Does ibc.GetPositionSize support fractional shares?

I don't think it does. It seems to just truncate the fraction after the integer, so 0.9 shares would be 0, and 6.9 shares would be 6.

IBController is open source. Everything is easily verified by looking at the source.
If you look at the source:

you would see that return value is integer.

And it is cast to integer using (int) so, it truncates any fractions.

Perfect, thank you Tomasz!