Try looking here? http://www.apple.com/ipad-mini/specs/
1024 x 768 for full screen. Same as the first generation iPad.
FM 12 has a template for that size - 1024 x 673 (landscape) and 768 x 292 (portrait)
The 1024x768 size works just fine.. Thanks to FileMaker using FM Go you can use finger gestures to do a zoom or reduce to allow your user to see exactly what they need to view.