The manufacturers charts take that in account.
The most in depth heat loss study is going to come up with the exact same answer 50-80k.
Since heaters come is 45/50k, 75/80k take your pick.
The manufacturers' charts are as broad as can be. The "heats up to" numbers are really irrelevant. That's the purpose of a manual J calculation. That said, a shop is pretty simple to heat. A shop will typically need higher BTU per square foot than a house would for a variety of reasons.
For example, I live in a warmer climate than the OP does. I installed a 45k BTU Mr. Heater in a 30x40x10 shop. It could heat it but had to run a long time to bring the temp to where I wanted it. I don't heat it full time. If the heat has been off for a couple days the temp in the shop will drop to around 45-50. I want to be able to go out there, turn the heat on, and start working and feel the temp come up quickly. The 45k couldn't do that. So, I removed it and installed an 80k. The 80k is a little overkill but does what I want it to...which is what's important to me. Plus, I have two 9 foot tall/9 foot wide doors. They are insulated but still not nearly as insulated as a wall would be. So, there's a huge heat loss when I open the doors. I want to have the temp come back up quickly once I close the door.
The OP hasn't provided enough info to make anything other than a guess at what size he might need.
There are also other sizes than just 45-50k and 75-80k. I've seen vented heaters as low as 30k and higher than 150k. There are many sizes for the OP to choose from. Without knowing more about his install, I would estimate the smallest size he should consider is 80k BTU and he might want to go even larger.
