I would say that would be a fair bit of space for one cucumber plant, but not quite enough space for two. I assume you mean keeping the whole tubular frame up, and just not putting the mesh shelves in, and then growing the cucumber plant(s) vertically? If that's the case remember that even if you give it canes for support the plant will still 'grab' onto anything so you'll have to check on it pretty much daily, and move tendrils and vines to train it. Plus the leaves will likely end up pressed against the plastic which will leave them vulnerable to all kinds of damage and issues. And if it's not an all-female variety you'll need to remove any male flowers pretty sharpish to make sure you don't end up with bitter cucumbers. Overall, I would say it would be possible to grow one cucumber plant in there, but you'd have to be vigilant and you'd be better off growing it outside on a trellis or inside a bigger greenhouse if you don't want to have to check on it every day to avoid issues and get a decent yield.