Minimum Room Size (w/o walls blocking view of avatar)

I notice that when an enclosed build is too small I sometimes see the wall in front of me instead of my avatar (unless I change to mouselock) . Before I do online experiments I thought I'd check here. What's the minimum room size to use to avoid this problem?

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The 'chase' view camera is by default approximately 2.5m to the back of the avatar I think. But you can just use the mouse scroll wheel to adjust this quickly. You will still get the wall sticking in your face regardless of the width and length of the room because it happens when you walk near the walls.

For the height of the ceiling, go no lower than 4m.