Sessile organisms can get pretty big. The largest organisms in the world, by mass and area, are a 100+ acre aspen grove in Utah and a 2.4-mile-wide honey fungus in Oregon.
Of course, those are sitting in or on the ground. It gets more complicated when you have to suspend most of the organism as a roof up in the air. Since these things are holding pressure in, though, the mass of roof can in fact be held up by the air pressure inside. In fact, I'd expect these things to grow so that the areal density of the bubble material produces weight that exactly balances air pressure, thus minimizing tensile load on the structure. That way, in ideal conditions, the structure could grow arbitrarily large- even encompassing the entire world.
You will, however, need to deal with dynamic instabilities in the roof structure, and since this is a living organism, you'll also need to worry about transporting nutrients throughout the structure. Both of these constraints mean that the size of your balloon-dome creatures will be restricted by their internal structure; do they have tendrils or columns or something in the interior, which could be used to anchor the roof, damp vibrations, and transport water and food from the ground? Or are they anchored and supplied solely from the outer edges?
If they have internal support, there's no reason they couldn't get arbitrarily large. Otherwise, the precise limits are going to depend on details of the material strength of the organism's tissues, and the efficiency with which is can transport water and nutrients. Anchoring area at the edge grows linearly with radius while the area of roof tissue to be fed grows quadratically, so eventually that will catch up an impose a maximum size (fortunately, you're not limited like trees, since most of the transport will be horizontal, rather than vertical, and the lower gravity helps with the vertical component). You'd pretty much have to make up the precise numbers, but personally I'd find a football-stadium sized enclosure to be entirely believable.