how do you change the screen size when mirroring

Mirroring won't fill the TV screen. The iPad has an aspect ratio of 4:3 and the iPhone 4S or prior an aspect ratio of 3:2, neither of these will fit exactly into a TV screen which as an aspect ratio of 16:9. The Apple TV will not zoom the mirrored image, because it may well cut off information which is situated at the top or bottom of the screen.

You may wish to try each of the settings in ‘Settings > Audio & Video > Adjust for AirPlay overscan’ for the best setting that suits you.

as stated then mirroring a non widescreen device (mirror being the key) to a widescreen display will result in it not being full screen

or cut top and bottom of the image being displayed

but it is leaves bar on top as well as on the sides then it's something you have to change using your tv's remote not the appletv