Get the biggest one that you can squeeze in and at least a size bigger than you can make plans for or sensibly afford.... also look at ebay, the bigger ones frequently go for the same prices as the smaller more popular ones.
Size matters, the bigger they are the easier they are to thermoregulate, so also look at plenty of vents./..
I ended up with a 20x10 I bought secondhand and it owes me around 400 quid I think, thoguh I might get some back selling some of the spare vents as it was used as an alpine house so it was almost all vent.....
Auto-watering and autoventing are very useful, buy them in that order, a timer control on the watering or fill a water-butt adn let it drain through....