For the layout's width, I've tried writing
Stage 0 Width
"Stage 0 Width"
but they are not accepted.
How do I use global variables in this case?
For the record, if you are using it in an expression, it would be:
global('Stage 0 Width')
Otherwise, if you use the Compare global variable system condition, you just select it from the drop-down list.
Note the use of the single quotation marks. They are reserved for identifying variables in Construct. Also, be careful of using whitespace in the names. Make sure that you don't have extra spaces or whatnot... I prefer not to use spaces in names (i.e.: Stage0Width.) Not sure if I've ever tried it, actually.