Is there a maximum imposition sheet size?

There is no specific limitation in eDocBuilder regarding the size of the imposition sheet.
Besides, the maximum dimensions for Adobe PDF are 5 x 5 meters maximum (so you can't create a document larger than that measurement), as the processing of imposition sheets is done by the same means (i.e. ABC pdf library), the limits should be the same.
In any case, it is important that the measurement system chosen is the right one to enter the format correctly: