The stage size is an editor option and so is one of the options for the getters and setters library, but it uses javascript (and jens is probably gonna remove it). But you can actually set it using SciSnap! primitives.
Run this block to load the SciSnap primitives, don't delete it
.
[scratchblocks]primitive[src_load(url) v][libraries/SciSnapExtensions.js]@delInput@addInput::grey
primitive[SciS_setvalue(which,value)][Stage size](list(width)(height)@delInput@addInput::list)@delInput@addInput::grey//add me underneath[/scratchblocks]
If the primitive blocks don't appear in the pallete, tick the extension blocks setting. If you don't know how to get it JS-less, Check [this topic](https://forum.snap.berkeley.edu/t/stage-size-without-javascript/9621) by @loucheman .