Here are sizes for most roll pins. The forward assist has three roll pins. The one that actually holds the assembly in the upper is 3/32x5/8.
A1 Rear Sight Wheel Retaining Pin.................1/16x1/4
Trigger Guard Detent Pin.................................1/16x1/4
Forward Assist Pawl Pin(round)......................1/16x1/4
Forward Assist Pawl Pin(teardrop)..................1/16x5/16
Ejector Pin.........................................................1/16x3/8
Gas Tube Pin....................................................5/64x5/16
Charging Handle Latch pin..............................5/64x5/16
Bolt Catch Retaining Pin..................................3/32x1/2
Forward Assist Pawl Pin..................................3/32x1/2
Forward Assist Retaining Pin..........................3/32x5/8
Type D Stock Swivel Retaining Pin.................1/8x7/16
Front Sling Swivel Pin......................................1/8x1/2
Trigger Guard Pivot Pin...................................1/8x5/8
Rifle Buffer Pad Retaining Pin..........................1/8x3/4
Type D Stock Swivel Bracket Retaining Pin....1/8x7/8