This is a very relevant topic - particularly at this time of year when rents are due (well they are on our allotments.) Our society doesn't push new members into taking on the old style allotment plot _ 300 roods perches or some other kind of fish - but splits up the old size into roughly thirds. Most newbies want to emulate Monty Don etc, but don't appreciate (or can spare) the necessary time to keep on top of things. An allotment can be a very therapeutic thing, but not if you are constantly fighting weeds, slugs, birds etc, and then finding you have fallen behind with planting out the module raised plants, or sowing in the right time for best results. I would suggest taking the smallest size plot you can, mastering that comfortably, and then moving on to something bigger if you enjoy it and can afford the time. The first year is always the hardest, but stick with it and it gradually improves as you learn more about your soil and environment. Most of all - enjoy it!