The test seats are there for people like you to use so don't feel embarrassed. That is what they are there for.
You know you are big, you have freely admitted it and I'm sure nobody would think any less of you for doing what is totally sensible. It will take one second and then you will know. Everyone else there is doing their own thing and having a good time. If they are going to think anything of you sitting in the test chair then you can be damn sure they're thinking it anyway when they pass you by. Sitting in the test seat isn't going to make them realise you're big, you're the same person outside of the test seat that they would have walked past or stood next to in the queue or wherever. You know, I know and everyone else who sees you knows...and it means nothing. You are the way you are because you want to be and that's fine.
It'll be like ripping off a plaster. Just do it - quickly. No need to measure yourself or make yourself feel bad if you don't ordinarily feel bad. It will take a few seconds and it'll be a yes or no answer.
Judging by lukaNW's post, you'll be fine anyway.