Sub Panel in detached 10x10 bldg(welding shed)
You are on the right track. What is important is the duty cycle of your individual welders. NEC Article 630 is all about welders including there duty cycles. 

The duty cycle is determined by what type of welding you are going to be doing such as small projects that are never more than 1/4" steel and the type and size of welders you will be using.

I suggest you try and Google or find a copy of NEC 630 and do a little reading before determining your service size to your building.

Good luck.