Normally 1hp for every foot, so anywhere from 4-5hp will be ample.
First thing is whether you want 4-stroke or 2-stroke. Advantages with both. In addition, fuel supply. If you hook up to your main fuel source, then that would be ideal.
2-stroke, light, easy maintenance, and normally bullet-proof unless its a 5hp Twatsoo!
4-stroke, quiet, cleaner, more frugal, but watch how they're stored (regards oil in sump)
Also how will it be mounted. If there's sufficient space on the transom, then that would be my preferred choice. Adjustable outboard brackets rattle like a bag of spanners.
The other main thing to consider is the robustness of the outboard when underway. The tilt pins on most outboards won't put up with the vibration, so considering tethering with a ratchet strap. You could use a stainless steel u-bolt, that's what I use straight into one of the transom knees as an anchor point.
Shaft length, you'll know your own boat and the freeboard at the transom. I chose a long shaft 4hp Mariner and it clears the water when underway.
Regards getting of a beach, still think you could use you main engine at a trimmed up angle, suppose it depends if you have a stainless steel propeller (danger of hitting the bottom) and what the sea condition is like.
Plenty threads on auxilliary outboards, for pictures, but holler if you want more details.