I believe I had troubles with the "sub(A)" part as well, let me upload the actual program, see if it works

But there the maximum number is 65535 (serveral times pressed 9, or the [Y=] key )

Axe has a maximum number, there is little we can do about it.

The ideal input routine should store the input into a buffer in ram.  Then, the programmer has control of how to interpret the buffer; either as numerical, string, or something else.  And yes, you can store numbers larger than 16 bits using part of RAM to store the larger number (kind of like how the 16 bit number is really just two 8 bit numbers).  In fact, you can store as much precision as you want.  You can have a 100 trillion digit number if you need it.  It just complicates the math you have to do, but little subroutine can take care of that for you so you don't have to deal with it manually.

With some thought, it is possible.  Unfortunately, I am low on thought right now.

I can get to work on this, along with the other input routine. These will most likely be needed eventually anyways

I made a 16x16 sprite drawing routine, and a program to help make the sprites.  Here they are:
for the sprite making program, just use the arrow keys to move, and 2nd to toggle a pixel, then Clear to convert the sprite

Edit:The first sprite editor was not very optimized, this one is
Edit2:there were a few bugs that made it either quit or not draw the sprite correctly (order of operations messed it up), anyways heres the new "Dspaxe16" program

What's the size difference between unoptimized and optimized?