My Veg patch at home is 266 square yards, what size is the average Allotment, or do they all differ.
Some allotments are nothing more than a plastic bag...they up from there depending on need, capability and what you can get!
Basic rule is; get what you can area wise and work what you are capable of managing.
That's quite a good size some of the allotments on my site are smaller than that, mine is 334 square yards....but your're probably not including your shed as part of your vege patch are you?
I'd be happier having a vege garden that size at home rather than an big allotment. At home any free half an hour can be used, there are no vandalism or theft problems. I wouldn't have to worry about weirdo's creeping up behind me when I'm working as we don't have any security fencing and sometimes I'm there on my own.
This link has many different sized plots on it at different prices too.
http://www.allotments4all.co.uk/smf/index.php/topic,37173.0.html