So, I want to create a world (like a big one -Tamriel Sized-) and I was wondering if anyone could
offer general advice that I would need at any point in its construction (possible errors, things to make making it easier etc.)
I really want to make this world and I would greatly appreciate any advice anybody would have to share.

I appreciate it

read the tutorials, keep lots of backups, and be patient. have a good plan of what exactly you want to make.

A few things that come to my mind:

1. Make sure the LOD mesh will look reasonably good. It is very low poly compared to the detail landscape and a bunch of small, steep hills for example will become one large hill in the LOD mesh - doesn't look very good if you approach the LOD land and it suddenly changes from a large hill into several small hills when the detail landscape is loaded. It is best if you don't put too much height amplitude in a small area. Use soft slopes only if possible.

2. LOD water only shows at z 0. Avoid making large mountain lakes or something similar. Large bodies of water should only exist at z 0. There are workarounds to make water show as LOD at higher levels as well, but they look like workarounds and cost some performance as well. Can be used for small lakes or similar things, but not for a whole ocean or a really large lake.

3. Don't go overboard with the heightmap size. It is easy to make a huge heightmap and it is also easy to clutter it with the region editor. You can do both of that in a few days, and most of the time you don't even need to do something as you will be waiting for your computer to finish the regions (can take several hours or even days depending on the region size and your computer). But while a landscape created like that doesn't look bad it is boring as hell. All cells in a region look more or less the same, there is no reason to explore the land. You need to add and adjust many things manually if you want it to look really good, interesting and if you want to make it worth exploring. If you have 20+ quads it's simply too much for a single person or even a small team. 3x3 for example is already a very large landmass, but it is at least possible to adjust a good part of it by hand. I would never go above 4x4 (and even that is not really possible unless done by a pretty large team over a long period of time).

How big is Tamriel? Just wondering because I think the map I made is about that size.
Thank you for you advice too.

IIRC the Tamriel worldspace has 14 quads including the inaccessible border regions (Skyrim, Elsweyr etc). Cyrodiil itself (the area the game plays in unless you disable border regions) is about 5 quads.

Thanks for the help!