It’s really a decision of Shed vs Workshop, this. I wouldn’t insulate a Shed, frinstance, because it takes up a bunch of space and you’re not likely to ever get a massive benefit from it if you’re not in it a lot. Working on bikes or similiar, you still tend to be fairly selective about when you do it, you’re not real likely to be out there at 10pm on a freezing night like you might be if you had a big diy project or similar, and you just don’t need to worry about hot because you can move the work outside.
So it’s kind of, how much will you work in it and what’ll be the nature of the work, the timing of it.
But for sure go as big as you possibly can. Try and think about the spaces you work in already- working on a bike in my small kitchen absolutely sucks, it’s “big enough”. there’s always something in the way, you can’t rotate it or work easily around it, it’s always bouncing off the wall or knocking things over. Working in my living room is night and day even though teh actual work space isn’t massively bigger.
(I have a decent sized shed, for my lathe and other big tools and for storage… I’ve built an engine in it but I wouldn’t work on a bike in it, the space needs are so different)