1/4" is about as big as you want to go cold with a 4lbs club hammer, 3/16" may be easier if you can't get a really firm support under the rivit.
On these size rivits it does not really work to heat the rivit, place it and then form the head as there is not enough metal in teh rivit to hold the heat, it will have cooled by the time you have the hammer in your hand. If you have access to oxy then get a helper to heat the end when the rivit is in place and then form the head.
Threaded rivits are another option, just fit nuts inside and have the factory formed head on the outside