You can unlock a Cheat called 'Big Head Mode!' then enable it in the secrets menu!
N64 Location:[]
Go to Temple Of Prophecy (Marcher) and enter the "Flambeau maze". Find your way to the center of the room on the ground level and the "Secret Activated" message will appear.
Dreamcast Location:[]
Warp to Asylum: Playrooms and go to where the playroom cells are found (with a pool table and a playpen). Enter the middle empty cell.
PC and Remastered Location:[]
Go to the Cathedral Of Pain and drop down the shaft into the Marcher lava. Go on the top of the spinning blades, then find your way to another shaft that goes down. Drop down and the "Secret Activated" message will appear as you fall.