The document size AND zoom both affect the size of the final, exported image. You hit export in the document menu to save out that image. If you are at 50% zoom (AAHalf), the export will be half the size of an export at 100% (Actual).
So say you need your render to be 1000 x 1500, you would:
Resize your document to 2000 x 3000
Position your model, lights, etc
Hit AAHalf
Hit Best Render under the render menu
Document>Export
Viola!
Note that you can hit AAHalf, Actual, or Zoom after rendering to export different sizes, so it doesn’t have to be in that order. Also, you can export a canvas that hasn’t been best rendered. Good for progress shots when you don’t want to spend time on a render.
The larger the render your computer can handle, the better the final image has potential to be. However, it really just depends on the level of detail in the scene and how big you need your final image to be.
Try out some tests, you’ll get the hang of it 