Searching through 3d warehouse for doors and windows, I’m not seeing sizes. Is there a way to see this info, and/or are most sizes adjustable after downloading?
If you look at the page for the object there will be some dimensions of the overall component given but that will be automatically generated from the component’s bounding box and may not be exactly what you need. You can search including dimensions. There are also a large number of Dynamic window components as well as Live components so you can choose the dimensions. Or you can modify them to suit.
Note that to modify Dynamic Components you would need to be using SketchUp Pro. SketchUp for iPad doesn’t have the setting controller needed.