Just reading through this post, as I always tend to do the "weight/size restrictions" posts before I visit...I worry everytime as I am large!! 5"11 and uk size 18 right through to a 22 depending on where I shop, and although the original person who posted this has visited already and got on the rides fine I just wanted to put my experiences here for anyone else whos wondering...
I asked for the larger seat on Oblivion, however it felt too big around my chest but tight at my thighs but the "normal" size seat was too tight and i couldnt get the restraint down.
Rita tester seat - I could get the restraint down but couldnt do the belt up, however my friend who isnt fat but 6"4 got the restraint down and belted up but he found it too tight and far too uncomfortable so opted to sit that ride out.
Air - AS IF! Didnt even attempt it!!
Nor did I go on nemesis due to having to move the larger seat at thorpe park, and having a few people snigger at this. Though my friend did go on, and he looked like he wasnt even in the "normal" seat securely, and personally if i was him, would have requested the "big boy"
RMT - I got on the fine but because I am tall, I need to sit sort of diagonally alone with my legs to the side for my legs to get low enough for the bar to go down and lock. Though again my 6"4 friend who sat with his gf got in no trouble.
Thirteen - as above but just the other way round, I sat with my 6"1 husband with no issue's my tall friend had to breathe in and get a good push on the bar from the staff for him to fit in, he ended up so uncomfortable he wouldnt go on it again.
Hex - TIGHT TIGHT TIGHT, bit of shock having it down so heavy, so put my legs to the side familier to RMT just for comfort really, but no issue there! However my friend had tears in his eyes because it was so tight on his lap.
I think the point im making is I am bigger in width and height than an average woman, and I found things that my slim, taller than average friend more comfortable. As stated above test seats are there for the bigger people to try, and others seeing you in it may snigger, or make a remark, (as people do! I know this after being referred to as a 50 tonne gorilla in longleat by some sad indivudual that obviously set out to ruin my day with my kids) the brutal fact of the matter is as above you are still going to be big in or out any test seat, so just jump in!! Keep your head down or get the camera out pose for a big smiley thumbs up pic of you enjoying the test seat. Saves the wondering of "would i or wouldnt i have fitted" or the "walk of shame"
Just a long winded explanation of my opinion on test seats there!! I hope it helps someone!