05h AL$SIZE get an allocator cell size
Input X register - Tag of the cell to be sized.
Output D register - Size of cell in bytes.

Return the size of the cell whose base address is held in the two byte address pointed at by the X register.

If the value of X is not in the correct range ($2000-$203E) results will be unpredictable.

        LDX     CELL    ; get the tag
        OS      AL$SIZE
        STD     CELLSIZ ; Save the cell size
Errors - none -