If you are ever able to make it to the KSC visitor complex in Cape Canaveral they have mock-ups of both the Gemini and earlier Mercury capsules you can get in as a size reference. They are both incredibly tight. It's amazing during Gemini 7 they spent 14 days crammed in the capsule testing systems, doing EVA activity along with normal human activity (eating, sleeping, bodily functions). All while being seconds from death at any time if things go wrong. These early astronauts were men of a different caliber.