The size you want really depends on how much you would like to grow, a few people down the allotment have similar to your picture but they are half the size, a chap I know has one that is approx 12ft long, he has had it for many years with no problems, it has two doors which during hot spells can be left open for ventilation, however careful thought will be needed on where you position it so it is not very exposed to the prevailing winds, this poly tunnel I mention gets full sun but nearby are two sheds which give good protection, also that old saying you only get what you pay for applies, I have seen folk on allotment buy these cheap flimsy things that don’t stand up to the recent winter storms we have had over the years.

I had one of the cheap Guardman tunnels. Seemed a great idea at first but the cover was its let-down. Low grade polythene and such a narrow skirt at the bottom that it was not possible to tighten the skin properly; the loose cover causing it to flap about in the about in the wind , further weakening it.
I think that the main requirement in a polytunnel is that you get the skin drum tight and hard so that the wind does not bother it. I believe that with the commercial tunnels they get the cover as tight as possible and then jack up the frame inside it.
I would have concerns about being able to roll up the sides as this would indicate that they are not hard an tight when rolled down.