That's sounds do-able, if relatively pointless and time-consuming for no reason.
It's easy enough to have have a brick that sets player scale when starting.
Of course the people will start out large to begin with, and you don't want that distracting others, so why not build a little box far off out of the way. Then inside that box, place the Playerspawns.
Once the person spawns at full size, they click a button inside the box, which proceeds to set the playerscale as well as teleport them to the miniature area.
Of course you'll need "Setplayertransform" events to do this, but it should work and would save you requiring a completely new edit to the Mini-Game Gui, as well as keeping your miniature area clean of giant people.