Realistic level under 100 MB

I am building a level with very realistic graphics but I must keep the size of it under 100 MB. The less the better.

  • The player can only visit approximately 100 square meters in a first person perspective.
  • There is an empty house in the middle of the level. Empty means that there are really no objects at all, just walls, windows and stairs. Nothing more.
  • The house is in the middle of nature so there is some vegetation.
  • The player should see very far outside his moveable area.
  • Optionally there is a river.

Which techniques would you suggest to keep the size under 100 MB? Is it even possible?

I am very grateful for any help !