As far as I know, the scale you can go to is dependent on the scales available in the basemap you're using.
If you copy a tile package to your device for offline use which has images inside for 1:1,000,000 scale, you'll only be able to use the map at 1:1,000,000. If the tile package has multiple scales, you'll be able to zoom in and out to those scales.
If you choose an online basemap to take offline, then you need to choose the level of detail after you choose the work area. Then I think you won't be able to zoom in past your chosen level of detail.