I'm guessing you're making 'skins' of some sort. Other knockoffs or clones may vary. The lengths below are the full battery, not just the colored part, but If you measure the top part of the one you have and subtract from the size given - it should get you there. If you're doing it for the Twist, then you'll have to not cover the 'dial' at the bottom. That area is 3/16 inch wide.
eGo-c:
Diameter: 14mm (.55inches);
650mAh - 75mm (2.9528 inches)
900mAh - 90mm (3.5433 inches)
1000mAh - 100mm (3.9370 inches)
eGo-c Twist:
Diameter: 14mm (.55inches)
650mAh - 98mm (3.8583 inches)
900mAh - 110mm (4.3307 inches)
1000mAh - 121mm (4.7638 inches)
To go from decimal to fractional use this chart:
Inches and Decimal Equivalents
The number left of the decimal is going to be the same ;-) eg. on the l000mAh Twist - it's 4.7638 inches. So 4 inches then the .7638 is closest to 49/64ths.
Again, these are 'joyetech' sizes taken from the Joyetech site. Clones and knockoffs are likely to be different. 1100's and 1300's are not Joye products so you'll have to check other sites for those.