We don't know how big it is and I never i knew the exact size, based on its appearence it's most likely a pocket dimesnion, which is a one dimesnional space, and are usually small. city sized, island sized or bigger, we can't obviously quantify the exact size of course ...
Usually normal dimensions are practically quite large and overlap each other and some often distort space, time, gravity and other things...and range from city sized to millions of light years in terms of size even multiple galaxies...which is only relevant in fictional verses ofc..
Whereas The official scientific definition of dimension is an axis for which things can exist on - for example, a point is 0 - dimensional, a line is 1 - dimensional, a plane is 2 - dimensional, a cube is 3 - dimensional, etc. Time is considered to be the fourth dimension. According to M theory their exist 11 spatial dimensions which are curled up into small objects which we can't interact with. This idea is very controversial in the physics community due to many reasons and isn't accepted yet...
In fiction whereas, this isn't relevant, the hyperbolic time chamber is also a dimension, and do we know how exactly how big it is? Nope, it's hard to quantify.
Seeing how it is it's possibly a very large dimesnion...maybe planet level? or more in size based on how it looks, but to quantify its exact size? It's hard, usually some verses often state the size of their alt dimensions..
In other fictional verses like tenchi Muyu there are 3d dimensions, Multi universe sized, overlapping each other, which Kami tenchi almost destroyed by ascending...if dimensions were infinite, then many verses wouldn't be able to destroy collection of them as easily as they could lol....