Tip Size for IFT Measurements
The tip size does not directly affect the measurement, in the sense that the algorithm in no way depends on knowing the tip size. However, the size of the tip does affect the ease with which you can form a satisfactory pendant drop.
A tip that is too small will have difficulty supporting the drop. The drop will either fall off or climb up the needle.
A tip that is too large will necessitate a much larger drop in order that gravity distort the drop. However, this is normally not a problem, so there is less down-side to having a tip too large than too small.
A good place to start is #20 gauge needles (0.914mm outside diameter), but #22 (a smaller needle at 0.711mm) is useable for many liquids.