To the Right of the Builder is the Properties Panel. This is dynamic depending on the screen/layer you have selected and will automatically change accordingly, bringing up settings for the current Tools/Components selected.
When you first go into the Builder, the Properties are for the blank canvas on screen (or anytime you don't have a layer/tool/component selected:
- View/Change current Width/Height – of your canvas, or the asset you have selected. Click into the Width and/or Height field to change to the desired px: