Creating Custom Component + Pattern - Diptrace
The "SOT" is a "Small Outline Transistor", with 2mm distance between outer pins on one edge.

The "3" portion refers to the number of pins on the package, so an SOT23 is a 2mm small outline transistor with 3 pins.

The SOT23-5 is actually an incorrect designation, and should really be SOT25, though SOT23-5 is commonly used.

In this case the -5 refers to a 5-pin version of the SOT23, which would be a 2mm small outline transistor with 5 pins, which by the above numbering scheme should be SOT25.

Chances are you will have a generic SOT23-5 or SOT25 footprint in Diptrace since it is one of the most common size of surface mount transistors / small devices around.

If you do need to make a new footprint you shouldn't be working from the chip's dimensions, but from the provided recommended footprint or "landing pattern" in the data sheet. If a footprint isn't given, and it's a standard size like SOT25, then they assume it will fit any generic SOT25 footprint.

By the way, I use TI's SOT25 LM(something) regulators all the time (I forget the number off hand, might be the same ones), and just use the default SOT25 footprint in my PCB software and they work perfectly.

I haven't used Diptrace, so I don't know how to create footprints in it.