It is possible, but I don't recommend it. Making a city map on an overland map adds a lot of small details in one spot. This tends to just look like a mess when you are zoomed out to enjoy the overland map itself.

Additionally, city maps tends to get a bit heavy, so adding multiple of them to the same map isn't a very good idea performance wise either.

And you will need to do some manual tweaking regarding sheet setup, since overland maps doesn't have the same default sheet setup as city maps.

Instead, I recommend you use a regular city symbol and map the city as a separate map. Add a hyperlink to the city symbol instead, so you can click it to go to the city map.

A lighter and better way of making a city on an overland map is to actually combine multiple overland city symbols. This doesn't give you the same fine control over houses and streets, but it still look like a city, similar to these maps here:

As for your actual question, have a look at this FAQ topic:

If none of that works, you should consider contacting support. They can hopefully help you out. You find the tech support form on your registered user pages.

Uhmm..... why? Enlarging the image after exporting it won't add any additional details. That's just extra work from you for no benefit, and extra storage/bandwidth used by everyone. Normally people do it the other way around, exporting it larger than they need then reducing it, because this can help produce a better quality image than exporting directly to the resolution.

The fill style is set up to tile to fill without a target scale. This basically means it is mapped pixel to pixel to determine how many times it repeats. The tiling isn't a huge issue on the screen, but when exporting to a high resolution, there are many more pixels, so it will tile many more times.

Click the fill style indicator on the status bar, go to the bitmap files tab, find the fill (CA22_Starfileld 06 Bitmap I believe), and place a check mark in the "Scaled" option and change the widht and height values to 200.

(If you don't like how often it repeats, you can go back in and change these scale numbers. The higher they are, the less often it repeats, but if the numbers are too high it may cause the image to be stretched further than it's actual quality, resulting in pixelation.)

Just to throw a cog into the wrenches here. All atlas maps are always exported without the grid, because that often makes problems for people using VTT's, which is one of the main reasons to download the rendered image version. But then, referring to grid size becomes meaningless. So atlas maps do need an actual scale bar of some sort, but a tiny one is more than enough for maps such as these, doesn't need to be any larger than a single grid square.