If you buy the wood at Home Depot they will cut it to whatever size you need. You just need to find someone in the lumber department and tell them the measurements. There isn’t any charge for that. I don’t know about Lowes
I’m pretty sure Lowes charges by the foot. From the times I’ve gone I just picked out what I needed, had it cut to size, then they measure the wood at the register. I’ve never been charged for having lumber trimmed at Home Depot or Lowes.
From people’s answers, I’m thinking that the policy may vary store to store?
At the Home Depots I’ve been to, you don’t have to purchase the full size board.
Rather, it’s sold by the foot. The first couple cuts are free and after that, they charge.