When you export the image, there should be an option near the top that says something to the effect of:
Pixel Scale
(May be different depending on language)
Changing the Pixel Scale to a higher number will export it as a larger image. For example...
This is the unaltered version.

This one was exported at a scale of 6
The best place to seek a guide when it comes to ideal sizing is where ever the art is going to be displayed. For example, Youtube has a few guides on image optimization (sizes and such) for banners. Keep in mind that pixel art is best scaled in whole numbers. So you would want to scale an image to 200% rather then 150%. Starting with a larger canvas grants you more opportunity to put in detailed work... but can be daunting for starting out with pixel art.
(p.s. sorry about the horde symbol, was the smallest/simplest image I had available to use as an example.)