I think passenger volume may be a useful metric here. Below is what I found on a couple sites. Model 3 - 97 cu.ft. Current Accord - 105.6 cu. ft. Current Civic - 97.8 cu. ft.
I"m 6' 1" and plenty of room for me up front, and my 6' son can comfortably sit in rear seat behind me in M3.
I'm 6'5" and about 300 lbs. Besides a slightly narrow seat, the Model 3 is amazingly comfortable. I can almost move the seat back enough to not touch the end of the floor well with my feet. I can sit behind myself in the backseat with my normal driving position with moderate comfort except the sloping roof glass is just a few inches low. The open cabin really makes it feel larger than it actually is.
I just drove my mom to test drive the civic and accord in my model 3. The accord is super roomy. Rear leg room is ginormous even with the front seats set all the way back. The civic sedan is about the size of the model 3. I’m 6ft. A taller person would not find it comfortable sitting right behind me. The civic hatchback has the least room in the back. But if you only care about the drivers room, it’s more comparable to the accord.
Yep. It's an old trick of the legacy manufacturers: when there is not much change in the new generation -- make it bigger ! And since Americans are getting fatter by the year, the game plays well with the consumer.
every car of note does this. The 2020 Jetta is bigger than a 2000 Passat. 2020 BMW 3 is about the same size as a 2000 BMW 5 so on, so forth
Here's my car next to a Model 3 http://infotime.com/images/model3vsaccordsize.png posting link, also posted as inline image but forum wouldn't display that, don't know if I'm too new here or what.
You can throw the cars into cars.com and compare. It seems like a mixed bag. As others mentioned, rear seat: headroom superior to Accord, hip room closer to Accord than Civic, leg/shoulder it loses to both. In front, head/leg room it wins over all three, hip/shoulder room it loses to both. https://www.cars.com/research/compare/?acodes=USD10HOC021A0,USD10TSC031A0,USD10HOC011A0 As others mentioned, probably sitting in the real car is more telling, as raw numbers don't always tell the whole story on which car feels more spacious.
the model 3 replaced my 2007 Acura TSX (essentially an Accord) and the model 3 interior is definitely larger than my TSX was particularly rear seat leg room.