need to know size for channel balance.

Thanks for posting. Based on this balance's appearance and the 14" channel length, this might be the S351-14 or S390-14. That said, the fittings on your old balance are pretty banged up; I'd like to see a more intact balance, either the other from this window or from an identical window in the house, just to be safe. Assuming this is indeed either a Series 351 or Series 390, the only difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel' to 'toe'. See the diagram below for reference. Compare both with your old balance to see what matches best.

Comparison of the S351 and S390 balance shoes.

You'll then need to choose a stamp option. This designates the strength of the balance. Look for a stamp on the metal channel. If there is none, or you're unsure, weigh the window sash that these balances support. You can then match that exact weight with one of the weight ranges listed alongside each stamp option under the product description of the balance shop page.