Largest direct ancestor of humans
We can't really say for sure, because we have insufficient information about this. We can't trace every step of evolution through fossils. We can make educated guesses for most stages of evolutionary history, but they remain guesses.

Please note: Most of what we find in the fossil record are species that are not directly our ancestors, but rather branched off our evolutionary line and then went extinct. Those fossils can still give a good indication as to what the morphological features of our common ancestor were. And a lot of this is guesswork with some evidence behind it.

Going "down" the timeline of humans, we roughly encounter these steps:

Homo

The genus Homo: appeared roughly three million years ago, includes species such as Homo erectus and Homo habilis. Fossils of Homo habilis point to a height of around 100 to 135 centimeters, fossils of Homo erectus are closer in size to modern humans with 145 to 185 centimeters. The rest of the Homo genus was smaller than modern humans (Homo heidelbergensis/rudolfensis/neanderthalis)

Australopithecus

The fossils from Australopithecus are around 3 to 4 million years old, the most famous is probably "Lucy", a member of Australopithecus afarensis. Since members of all Australopithecus species appear to be smaller than humans (afarensis: 100-150 centimeters, africanus: 115 to 140 centimeters, etc.), it is likely that our common ancestor was smaller than modern humans. (Our ancestors likely were Australopithecus, we just don't know which species)

Apes

The split between the line leading to modern humans and the line leading to modern chimpanzees occured somewhere around 4 to 7 million years ago. The clade is called Hominini. The split between those and the line leading to modern gorillas occured around 8 to 19 million years ago (yes, the dates are getting fuzzier). A fossil coming close to this ancestor may be Nakalipithecus nakayamai, however, we only have a fossil jaw from that species. 

Going back, we get to the split between modern-day humans/chimpanzees/gorillas and modern-day orang-utans. This is the "ape" family, Hominidae. The largest ape that we know of, Gigantopithecus, that grew to about 3 meters, is classified as an orang-utan. Note that this is not a direct ancestor of humans. Even if our ancestors were larger than modern humans at this point it's unlikely that we are talking about anything larger than a big gorilla.

Primates

Going a bit in the reverse order here: The first true primates evolved around 55 million years ago. Fossils from that time are about the size of squirrels. Humans are "old world monkeys" who first appeared around 40 million years ago - the fossils from that clade we know, for example Apidium or Aegyptopithecus are a bit larger, some as large as a dog. 

Primate-like mammals

The first primate-like mammals, called Plesiadapiformes appeared around 60 million years ago. We don't know all that much about them, but the most famous Purgatorius was the size of a rat or mouse.

Mammals / placenta mammals

Going back even further, things become even murkier, but early mammals were small. Placentalia, placental mammals appeared around 90 million years ago. They were small, arboreal (tree-dwelling) animals. Early mammals appeared around 160 million years ago and fossils we have from that time place them around the size of a shrew.

Now, is it possible that there were larger mammals in there somewhere, that then "shrunk" again? Sure. Just unlikely.

Therapsid

Therapsis are mammals and their ancestors, evolved around 275 million years ago. The most well-known fossils from around then are Tetraceratops insignis. All we have from it is a skull of about 9 centimeters size.

Synapsid

Synapsis are making things complicated. As you mentioned in a comment, there were some really big synapsids. According to the Tree of Life project:


  Early synapsids were moderately large (body length between 50 cm and 3 m)


Tetrapods

Tetrapods is the name for the clade that encompasses what we now call amphibians, reptiles and mammals - those animals appeared around 380 million years ago. An example of an early fossil from that time is Dendrerpeton, which was up to 1 meter in length. 

Vertebrata

Synapsids are vertebrates. Vertebrates first appeared more than 450 million years ago. The time period following that, the Silurian, is called the "age of fishes".  Those fish could get really large, some of their jaws being over one meter wide. That's really all the information I could find.

Chordates

Taking another big step back, vertebrates are chordates, a group that first appeared in the Cambrian, somewhere around 550 million years ago. Early fossils are mostly small, for example Pikaia is about 5 centimeters large. Yannanozoon lividum is around 3 centimeters.

Before chordates

Before chordates, we really don't know all that much, the fossil record is poor because the structures that usually fossilize hadn't even evolved yet. For example, Hiemalora is around 3 centimeters in diameter. Whether it is our ancestor, we have no idea.

And before that come the first multicellular animals, the first animals, the first eukyrotes, and the archae,...

In summary

In summary, while we can only make guesses, the history of animal size from unicellular organisms to us probably wasn't spectacular. We go from very small (pre-Cambrian), to small (Cambrian), large or small (fish and synapsids), around 1 meter-ish (tetrapods), smaller again (early mammals, mammals), a bit larger (primates), again a bit larger (apes), and then to "human-sized".