If I have two longs going on, lets say long1 = 1btc size and OrderId= long1 and long2 = 2btc size.
if I put a sell order for 1.5btc with reduce_only=true and origClientOrderId=long1 is it going to sell only the btc from long1 or is it going to sell the entire 1.5btc?
You are confusing orders and positions.
A one-way mode account may have only one position open concurrently for the same symbol. Both long1 and long2 will contribute to the same 3btc position. The sell order will also contribute to the same position, resulting in a 1.5btc position once filled.
The origClientOrderId is a client-determined order identifier used by clients to manage order identity as they see fit.