If you're worried about pinching but you want to try the bit out for a few rides first, consider putting rubber bit guards on (yes, it's possible to get them on big bits like gags and pelhams, just takes a sturdy doorknob or a bridle hook, a pot of boiling water and some muscles). Even if it does fit, I'd stick guards on any gag bit because the way they move can still pinch lips or pull a whisker. Vaseline can also work if you're just trying the bit for a while, but it's not a permanent solution - a properly fitting bit is.
Otherwise, most bits are measured between the two rings. If you've got another bit that you know fits, measure the mouthpiece between the rings and compare.
If you're looking to measure your horse's mouth, there are things like the BitFit, but I just use a piece of garden hose marked with bit sizes (5.5", 5" etc) as a template. If you actually measure your horse's mouth, mark the very edge of their lips on your measuring "stick", then add about another 1/4". Most horses will be fine if you round UP to the nearest bit size to their mouth measurement.
Any photos of the bit in question on your horse?