Define reasonable size....

I helped put up a small Poly-tunnel (14x30 ft), in a very exposed upland site in the Scottish borders. It's still there several years later, in one piece, and doing it's job very well. Get one specified for exposed sites, and it will work.

The best labour saving device going for growing under cover is trickle-tape, ie the perforated hoses you leave on the soil, and they trickle water to the plants roots. They will save you so much time and effort compared to watering by hand, and cost very little.

Northern Polytunnels (link below) are very good, and do a very good range of sizes. I would urge you to go for the largest one you think you might want. The cost difference between them isn't too much, and the extra space is wonderful, as it allows you to do so much more. Things like early salad etc, germinating seedlings to plant out in the garden, as wells the cacti and tomatoes mentioned earlier.