I start mine out on rats personally, but I breed rats so I have access to all sizes. As far as eyeballing, you want it as big around as the thickest part of their body.
yeah she started on rats but the place by my house says they only have ones that are good for a bigger Bp
the last mouse I gave her was a bit bigger than that but she got it down and didn't have a huge lump so I figured it was okay.