Change the fill style resolution settings. Try to divide the numbers by 4. That should fix the pixelation.

Some of that text is pretty hard to read. That red and brownish text need a much more powerful light glow to be readable.

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.