[this one was supposed to go out yesterday, but it looks like I got distracted].
A plausible size will always depend on what you're trying to do. If you want a physically-based simulation down to human scale, it's likely that you'll be very disappointed because the actual sim will need to run a couple of orders of magnitude higher than the scale you really want. I don't know of any such software. Fortunately, there is an out. A simple and easy out. First, I will pontificate a bit:
Cartographer is ultimately the art of abstraction. Every map is done for a reason and for a client (even if that client is you) and is fixed in a specific medium. It will deliberately leave out things that don't support the reason for the map and will deliberately leave out things that the client doesn't want in there and will deliberately leave out things that can't be represented in the chosen medium. What does that mean? If you want a map of a country to show safe places to spend the night, then you probably aren't going to include much in the way of detail about the troglodyte race's pictographic representation of their perception of human-hobbit race relations unless the only safe places to stay are where there's a strong troglodyte-human-hobbit presence in the form a chain of "Ug's HoTEL" across the land. Similarly, if you're going to be drawing the map on orc hide using angel blood, there's a very good chance that you'll miss out the strong iridescence that's only possible using pigments derived from the vent scales of an aboleth.

Knowing that you probably don't have fifty terabytes of memory (about what Wilbur would need to do its work at 1.5Mx0.75M pixel resolution) and aren't willing to wait on the order of a day or so for each algorithm to complete, then you probably should be willing to go for much lower resolution and accept that a lot of details are going to go missing. Wilbur is fundamentally a simple image processing program and human concepts like map projections and "units per sample" don't really figure into what it does for the most part. It also doesn't have important facilities like labeling or overlays (except in a very very crude sense) and you'll need to go to a third-party product for that sort of thing anyhow. What it does have are some algorithm parts that will generate plausible results if used carefully. Those tutorials I referenced above show the kinds of things that Wilbur does pretty well and you'll see that most of the results are missing some specific elements like lakes and all of the labels are done in other software.

You should be able to pump out a good-looking heightmap with Wilbur, but it won't have a lot of Earth features like plate tectonics unless you put those in there manually. Wilbur is pretty good at elaboration from simple masks (https://www.cartographersguild.com/s...ad.php?t=25874 and https://www.cartographersguild.com/s...ad.php?t=51936 and https://www.cartographersguild.com/s...ad.php?t=30343 and https://www.cartographersguild.com/s...ad.php?t=31901 are examples), but results will tend to be plausible only within a fairly narrow range of scales and resolutions. I have a tendency to keep the final resolutions to 4000 or less pixels on a side because I lack patience. Some folks go higher. What you start with will very much affect what you get out ( https://www.cartographersguild.com/s...t=33087&page=2 shows some abuse I did to someone else's map and how the quality of the input directly affected the quality of the output ).