You cannot determine the amount of, say, JPEG compression, sorry. (In fact, I don't even know what PS Touch uses by default. 100% maybe?) xD
My exported projects (into JPEG) average about 3 MB in size. (Of course, that doesn't take into account dimensions, amount of color, etc.)
Any chance you have desktop Photoshop? You could export the project as PSD or PSDX and open it there and use the "save to Web" command.