Currently, if you open an empty place file and immediately start a solo test, the default character type is R15.
There is no possible way to change that type to R6 until you explicitly publish the empty place file and then change it on game page properties on the website.
This is procedure is way too obscure to find and completely inaccessible if we did not wish to publish our work for testing in a game that has already been built strictly around R6.
There should be some sort of workspace or game.Players property for setting whether characters spawn by default in R6 or R15 rather than having it strangely done on the website side.
The default character type is UserChoice. You will spawn with the rig type you have equipped on the site.
That being said, there are still a few reasons to toggle R6/R15 within a place rather than a published game setting:
My game is UserChoice and I want to test both R6/R15 compatibility in Play Solo without repeatedly switching back to the site and toggling my rig type
I want to test R15 with my game in a UserChoice test server, as test players appearance defaults to R6 so I have to set their character appearance ID to someone with an R15 avatar