How do you change the default via dimensions in Altium?
You can set the default via through DXP->Preferences->PCB Editor->Defaults->Via 

For placing vias in interactive routing, you can press 4 and cycle through sizes of via- minimum, maximum, preferred or user choice (favorite). 

Shift-V during interactive routing allows you to select a favorite from a list- and you can customize the list through DXP->Preferences->PCB Editor->Interactive Routing->Favorite via sizes