For an idea on what big projects can look like you can check out https://www.mendix.com/our-customers/. The actual answer to your question is something probably only Mendix itself has an idea about.
I have not seen the limits yet. I know that our model is quite large (about 250 MB model size). With a large model it will only take a bit longer to start up the model. And you need larger container sizes in the cloud. We work in a small team but I know there are also very big teams. With large team sizes it might be more convenient to split up the model over multiple applications but all depends on your architectural decisions.
Same as Ronald, have not seen the limits yet. I have been working on complete SAP / BPM replacements in the past at several clients, mostly replaced with multiple communicating Mendix apps.
Of course bigger apps come with other challenges, but it is definitely feasible!