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Hi Matt - Hope you enjoy the Allotmenteering game... Warning - It gets addictive.. Anyway on to your queries.. As Bill says they all have pros and cons..If you are willing to give it a good go and not scared of some "work" I would take the biggest one as Bill says. I say this as once you are established on a small one you may never get a chance to move to a bigger one. Also you may not feel like changing after putting time and effort in getting a smaller one to your liking then have to start again.Mine is long narrow (see www.gavinconway.net) which is normal on our site and others in town at different sites dotted around. Mine is approx 20mt x 9 useable space then a shed and greenhouse, compost heap (large) and storage space. This last season I grew 580 odd KG of veg on 165 sq mt!! I do have the greenhouse for toms.. The poly tunnel sounds a good deal but it's not the end of the world if you don't have one. You don't say what size it is.. You can pick up greenhouses free on freecycle or by putting shop cards up looking for one - same for a shed.. You can also get free stuff from garden centres / diy stores when they do refurbs. If you don't have time or energy then go for the poly tunnel plot and use it as a shed as well like you said. No need for a shed and a PT in that case. But as it is addictive you will have a shed and all sorts soon.... so get the big one and be done with it. Take a good look at my blog site and you will see that it can all be done on a shoe string if you are prepared to do some digging and asking around for stuff and keeping your eyes on skips - just knock and ask.. Have fun with it and don't make it a chore. Just do a bit each time you go to the plot and it soon gets done.
A sensible choice I think as it can seem very daunting with a large plot at first and can seem too much and then lose interest. You can grow a lot in a small space anyway.