What do you want to grow?

Have you considered Glass as well as a Tunnel?

The smaller the house the more difficult it is to get an even growing environment IMHO.

Glass with top vents and a louvre down low offers quite a nice atmosphere - the heat rises, and air is drawn in though the louvre, so I think has less "draft". Tunnels you open the ends, and the breeze blows through (but if you point the end at the prevailing wind then a gale may make it take off!). Glass normally has vertical sides, so easier to grow up to the edge, or to have a table/staging up to the edge.

Tunnel is cheaper (although check eBay for second hand greenhouses, bargains to be had) but needs the plastic replacing every few years.

Glass houses are prone to yobs throwing stones (Polycarbonate or toughened-glass [expensive] are possible solutions to that threat)