Interior Dimensions and Cargo Space of Tesla Model 3: What We Can Infer
Tesla has not yet released Model 3 interior dimensions because probably the final design hasn't been completed yet. I am expecting that the interior dimensions and other options will be announced either by the end of 2016 or sometimes in 2017 before Tesla starts the production so that nearly 400,000 people who have put their pre-orders may be able to configure their options.
What we can infer about Model 3 Interior
You would like to make sure that Tesla Model 3 will have comfortable room for at least 4 people. I think Tesla has claimed that Model 3 will have a comfortable seating for 5 adult people. It's going to be a car in midsize category. Toyota Corolla and Honda Civic are midsize cars and you can compare your Model 3 interior expectation to that of Toyota Corolla and Honda Civic. These cars do comfortably seat 5 adults. Therefore, I expect Tesla Model 3 will have comparable interior and cargo dimensions and even feel more roomier with the glass roof.
Tesla will first finalize Model 3 interior dimensions and then release them. There are some good comparisons of Model 3 Interior vs Model S and other midsize cars online that effectively show the interior of Tesla Model 3 will be effectively as large as the Model S -- which is a full sized sedan.
Since Audio A4 and BMW 3-Series are compact cars per EPA, Model 3 midsize is going to be bigger and roomier than Audio A4 and BMW 3-Series.
Those who are leery of putting and order for a Model 3, which they have neither fully seen nor driven, currently the requirement is only to put a deposit for a place in line. If you chose not to buy, the reservation deposits are fully refundable according to Tesla. I personally think that soon Tesla will have opportunities for the reservation holders to test drive Model 3.