I'm like you 225 and 6'3 and was serching for a sauna.
I ended up purchasing one of those portable ones - the one I got was a decent size. I mean it still felt a little cramped but I didnt mind using it. And I still sweated pretty well from my head using it.
I moved to the UK last year and ended up purchasing another portable sauna (this one is smaller) - and ya I dont like using it as much because I'm pretty cramped in there. But it still does the job well.
If I had the space and money etc I would probably get something where I could relax and lay down in. Else for the price and the convenience (can fold a portable one up and store it); the portable one isnt that bad.