The size of the decal shadow is automatically determined from the visual size of the object. So there is indeed no parameter to set an individual size, and the player frame has no influence. But you can use the shadow parameter of the entity to set an individual decal shadow sprite of a different size and shape. This should work in all A8 versions.

Do NOT use old versions. When you think that something is wrong with a certain version, please post a bug report with an example. Decal shadows are no different in A7 and A8, and we have no report of any problem with them in newer A8 versions.

http://manual.3dgamestudio.net/shadow_stencil.htm