CASA (Australia) have regulation dictating screen size etc. iPad mini is fine.
The other factor is that your airline needs to justify to its regulator that the EFB is safe to use and does not interfere with aircraft systems. I don't know about Boeing, but Airbus produce a document for each iPad type showing its testing and non interference with aircraft systems. For this they charge a few grand for each operator for each specific iPad model (ie ipad2, iPad Air, iPad air 2, etc). As such you might need to abide by what documentation your company has purchased which in turn limits you to what you could use.