Hi sbtlink,
UE should only keep textures that are used in memory as a general rule so any purging would just be to keep the disk footprint down.
As ClockworkOcean mentioned though, you can change the resolution that UE loads your textures into VRAM by changing either of 2 settings in the Texture Editor:
The MaximumTextSize can be specified as something like “2048” and the LODBias as an offset between 0 and 8 - telling UE to use the next size down (MIP) of the texture.
I have a tool on the marketplace that makes editing those settings very easy - you can select all the textures you want to change, right click and choose a LOD or MaxTexSize straight from the menu: