IMHO I'd say 3 inch if you can get it, if not 4 inch, easier to handle indoors. I'd also go for the 7 shot just to have one more loaded and they do make speed loaders for the 7.
If 357 is too much you can back it down to 38+P.
Here's my 686, seven shot loaded with .38+P stashed for HD where we can get to it quickly if the time comes.
