Yesterday I've discovered that when exporting any asset, I can type any number before the "x". So, I'm not limited to the predefined export presets. Neat!
Another trick I learned recently is that you can type the number of pixels followed by "w" for width and "l" for length to export assets at ANY size.
For example, if you have a an icon that is 150px tall by 100px wide, you can just type 75w in the export tool and Sketch will export your asset at 75px wide and scale the height accordingly.
In this case, you'd get something that is 75px wide by 113px tall. (Sketch rounds to the nearest pixel).
Proactive thinking by Sketch developers. awsome
You can also use half ratios like "1.5x".
I found this out because I based my iOS document at the @2x resolution so exporting for @3x I have to use 1.5x.
That's fine, I was expecting that.
It's really a forward-thinking idea! Lots of great Sketch tips here https://designcode.io/sketch