Keep 2d relative to 3d

Hello everyone, I am currently coding a game where the characters and such will be 2d, but buildings will be 3d.

Right now I use orthogonal projection for my 2d layers and it works very well, I have it setup to use the 0-800, 0-600 coordinate system. However the 3d layer does not use the same coordinate system, and if I say scale the window it scales the 3d while not scaling the 2d. I need the 3d to stay relative to the 2d (if that makes any sense).

What is the best method to achieve my goal? Ideally I would like to keep the 2d aspect alone. Is it possible to achieve orthogonal projection while still maintaining a 3d aspect? I don’t want a window resize to result in the 3d being stretched or anything.

Thanks for reading everyone, I hope I explained my situation well enough.

You should enlarge the view frustum.