Use a minor restriction of the LOOP language described under Wikipedia's "LOOP (Programming Language)". The restriction is to eliminate constants. So, the language contains increment: $x_i++$, decrement: $x_i--$, assignment: $x_i := x_j$, sequencing and loop.
The output of the program is the final value of $x_0$, and the length of a program is the total count of increment/decrement/assignment/loop. For example, the following program has a length of 8 and outputs 34. (I use x and y instead of $x_0$ and $x_1$.)
x++ y++ y++ loop y loop y y++ loop y x++
I have added the recreational-mathematics tag, as this question is similar to "Small Representations of 2016".
This question is mainly for fun, but hopefully more challenging/interesting than the usual questions asking to use certain digits.
As explained in Wikipedia all variables are implicitly initialized to 0.
Programmers should also find this challenging.
It is not an official contest, so I am not cheating!