Is there any confirmation, or speculation, as to the internal dimensions of the Model 3?
No, there is nothing confirmed from Tesla. Yes, there has been plenty of speculation though. I think Car and Driver had an article a few months ago, as well as few others. Google is your friend, just rember that nothing is definite and it is all subject to change until the next reveal.
You could take the Model S width (77") and the MotorTrend's Model ☰ estimated width (74") and assume the interior width will be 3" narrower than the S.
From most estimates/accounts/photoshop overlays the interior front-to-back space will be close to comparable to the S.