Length wise it still amazes me how Avon & Humber define "one man" & "two man" etc.
For example - The "3 man" seat on the SR5.4 I always feel has slightly too much space for 2, with 3 it gets comfy. Same comment regarding the "2 man" seat on the SR4. Likewise, my |"2 man Standard Jockey" on the Humber - if the bolster was about 6" further back it would be fine, but when I'm sat comfortably for the wheel, the second person is literally clinging to the back of the seat. I'm not particularly rotund, but the thing that kills it is the fact there's nearly 8" of cusion in front of my @rse - if the whole bolster was mounted 6" further back, there would be plenty of space for everyone. The foot straps are also rather badly sited, but I put the entire setup down to me being taller than the person it was designed for.
As for height off the deck, that's an interesting debate. Mine is "proper low", so when you are sitting, you aree most definetly sat. Standing, plenty of room to allow you to bend knees when required & not get the seat trying to break your spine - also makes it easier to get on. Friend with a Ribcraft - nice high seats - really comfy when sat, but not as easy to get your leg over*, and much less margian for error with regard to meeting it on the way down.
I'll second the comments on the width of the base.
*I do mean that literally!