Per the manufacturer, the size of the opening is about 4 inches in diameter, but the prongs can be bent inwards some if you need the hole to be smaller.
Tomahawk Rigid Single Door XL Fish Trap Easy Release Model 406 would not be good for allowing small mudminnow to swim out. We have spoken with the manufacturer and they suggested the Tomahawk Perch and Minnow Trap with Adjustable Funnel - Model 424. This it item is only currently available via phone order for $106.93
Per the manufacturer the opening on the Tomahawk Rigid Single Door XL Fish Trap Easy Release - Model 406 is about 4” diameter so up to that size.
The 406 Single Trap Door with Easy Release Door Rigid XL Fish Live Trap will hold catfish up to 5 lbs. The trap can hold up to 15 fish at a time.