Accepted answer
At 1 mg/mL a 1 mg dose is 1 mL — 100 units on a U-100 barrel — and no needle gauge changes that number. Gauge changes three other things: how long the draw takes, how much stays behind in the hub, and how much rubber you core out of the stopper. On the 25G scale a larger number is a finer needle, so a 25G drawing needle is coarse enough to draw quickly and coarse enough to cut a visible plug from the stopper. If you are drawing 100 units at a time, the dead space matters more than the bore: a fixed-needle barrel loses microlitres, a luer hub loses tens of them, and at 1 mg/mL each microlitre is 1 µg.
Answering this needs to know the viscosity of what is being drawn, since a viscous solution through a fine needle is slow enough to encourage bad technique.
Typical outer diameters: 21G is about 0.82 mm, 23G about 0.64 mm, 25G about 0.51 mm, 29G about 0.34 mm and 31G about 0.26 mm. The gauge number and the diameter move in opposite directions.
The underlying point is that drawing a viscous or foamy solution through a fine needle takes long enough that people rush the plunger, which causes more foaming. Using a wider drawing needle is the fix.
Needle gauge to outer diameter correspondence is standardised and published; the inverse relationship between gauge number and diameter is the reason for the counter-intuitive labelling.
Length affects comfort more than gauge does at these volumes.