How large is a neurotransmitter? Max and min sizes? What size(s) are neurotransmitters? What is their size relative to the synaptic cleft? Relatively speaking,  can we tell how far they must travel to cross the gap? Twice their own length? More?

I reallize this calls for gross oversimplification, and there are bound to be infinite variations, but I am building a conceptual model to understand relationship between the neurotransmitters and their respective "milieux".
From the textbook Neuroscience. 2nd edition:

Neuropeptides are relatively large transmitter molecules composed of 3 to 36 amino acids
Individual amino acids, such as glutamate and GABA, as well as the transmitters acetylcholine, serotonin, and histamine, are much smaller than neuropeptides and have therefore come to be called small-molecule neurotransmitters

Large proteins are around 10nm, because the amino acids wrap around each other.
This means that a good estimate of the size of neurotransmitters would be from around 0.5 to 5 nanometers.
Typical synaptic cleft distance is around 20 nanometers