Shaft diameters and keyway sizes in model IC engines?

In preparation for and hopes of my first IC model engine build, I have been drawing up some plans in CAD for how I might customize a Webster - customizing partly to suit materials on hand, partly to use ball bearings on the crankshaft, cam gear, and big end of the rod, and partly to use 20° module 1 gears (since I have the ability to cut that size, but not the 14.5° 32DP gears called for in the plans). I'd also prefer to use a key in the crankshaft, flywheel, and timing gear rather than the roll pins called for in the plans.

That last bit is the source of my question-of-the-day: For those of you who do your own designs or customizing of designs, what parameters drive the choice of key size and of diameter for the crankshaft? Obviously, the two are related - as I am contemplating the 5/16" (8mm) diameter crankshaft in the plans, I am doubtful about how small the keyway would need to be - 1/16", perhaps? Would that hold adequately, without overly weakening the shaft? And actually, that 5/16" shaft seems so small compared to the flywheel that it has me a bit nervous.

One option is to increase the size of the shaft and use a larger keyway - for example, I have used 1/8" keyways in 1/2" shafting before, and that seems to work just fine. I tried modeling this in my customized CAD drawings of the Webster, using a 12mm diameter for the shaft (because I have easy access to 12mm ID flanged bearings) and a 3mm keyway, as shown below. The keyway and shaft are right-sized for each other, but in the scale of the overall model, I'm concerned that the 12mm shaft is going to look too bulky.

Thus, I'm debating my options, and seeking input. Could I put a 3mm key in a 10mm shaft? Or maybe a 10mm shaft with a 2mm key? Or ... ??

Model showing the use of a 12mm crankshaft:

Google is your friend for metric or imperial key sizes. I use 1/16" key for 1/4" and 5/16" shafts, 3/32" key for 3/8" shafts, and 1/8" for 1/2" shafts.

Thanks - I shoulda thought of Google. That helped on sizing keys to shafts (though I'm surprised that the chart I found calls for a 4mm key in a 12mm shaft!)

What is your thought process when it comes to sizing the shaft itself? Obviously there is some lower limit under which the shaft is not strong enough. Is there a chart for such things? For the upper limit, is it just aesthetics, or there a rule of thumb, or ??

The majority of my engines have 3/8" diameter crankshafts and camshafts. My engines are typically 3/4" to 1" diameter pistons x 1 3/8" stroke.