I know this is a big question with loads of diferent considerations
As you say 'different considerations' these are mine;
I'm thinking 6x8 or possibly 6x10 depending
If area is your first consideration go for width rather than length, always remember you have to get into the greenhouse as well and when it is full of say tomatoes this can sometimes be hazardous.
I prefer 8ft wide to 6 ft wide for this reason
I've seen ones with 1mm thick poly-something rather than glass which sounds horribly flimsy,
I wouldn't touch this with a barge pole particularly if it is held in with spring clips. They tend to pop out and blow away in a high wind.
and then there's horticultural glass and touchened glass but I don't know what the difference is...
There is a big differeance particularly in price!
Horticultural glass is the standard for greenhouses and is 3mm thick. as opposed to 4mm thick you would get in a domestic situation,e.g. a window.
Toughened glass is usually laminated glass which is basically two sheets of glass stuck together and is generally quite heavy. Personally I don't think it is called for in a greenhouse.
On the same theme I don't like polycarbonate sheeting it tends to discolour in time and go brittle.............But them I am a traditionalist as this is one area where I won't go with new technology..........horticultural glass has been the choice of gardeners for generations and long may it continue.
As for a base, I'd like to be able to stand the greenhouse directly onto the soil
In a word.....Don't!!
You will get movement and glass doesn't bend when the greenhouse moves ..it breaks!!
what should I stand it on. I'm assuming I need a base of some kind whatever I'm standing it on though?
If you don't want to go into the realms of pouring concrete you can buy concrete blocks to form a base with. You can sit the greenhouse on this and plug & screw it to the blocks. As an additiond to this you could place a timber wallplate on the blocks and fix the greenhouse to this.
I have two Alton greenhouse and they supply purpose made concrete bases,