You have to point your liste of data. Eg :
:Data(2,5,100,40,3)->Str1
You can read those data in this way :
:{Str1}
:.Return 2

:{Str1+1}
:.Return 5

...

:{Str1+X}
If you want add a 2-bytes number, you can use ^r (2nd angle 3) :
:Data(12,500^r,200,30000^r)->Pic1

:{Pic1}
:.Return 12

:{Pic1+1}^r
:.Return 500

:{Pic1+3}
:.Return 200

:{Pic1+4}^r
:.Return 30000

Endeed a byte is 8 bits. Then a byte can take 2^8 (=256) values, that to say a number beetwen 0 and 255. A 2 byte number can take 2^16 (=65356) values, it means a number beetwen 0 and 65355.

Axe variables are 2 bytes number.

Do you know about anything about numbers bigger than 65355? Are they 3 bytes or 4 or something like that?

Combine how ? and what are OP registers?

Do you know about anything about numbers bigger than 65355? Are they 3 bytes or 4 or something like that?

Depends on how much bigger than 65355.

Basically, to know the limit of X bytes, you do 2^X*8-1.
So 1 byte stops at 255, 2 bytes stop at 65535, etc

The OP "registers" are a set of memory areas that are used as operands by the OS's floating point routines. Unless you need floating point, there shouldn't be a need to explicitly deal with them.
If you want to do math with larger integers, you basically need to write the routines yourself (precluding a library/axiom already having been written. I don't follow Axe too much.).
For example, to add the number BA (formed from variables A and B) to DC, and store the result back to BA, you could do something like A+C->A<C+B+D->B, where -> is the STO> symbol.
An easier-to-follow variation goes like this:
A+C->A
If A<C
B+D->B
Else
B+D+1->B
End