Steam injection boiler size?

[Alchemist75]

I'm contemplating building a steam injection set up for my main boiler for stripping mashes. My main boiler holds 4 gallons practically though I'd likely have to use smaller charges for steam injection, maybe 3 gallons. How big of a boiler would be needed for a steam injection set up given my primary boiler volume? Would a gallon suffice or does it need to be bigger?