I'm using 1024x1024 on my skybox.
Bigger than that is a waste of RAM and processor speed (The textures are loading into memory, then the poor CPU/GFX card have to reprocess them and spit them on screen), since you are planning to stretch it out over a fifty story brush, you could probably easily use a smaller repeating brush. Also - are people really going to be looking at this texture at all. I mean there's no point putting in a huge texture if people will only see it a few times or from a certain angle. Better to keep the Hi-res brushes where people can see them and use lores ones for places the players don't really go.
That said the only way to really test out the size limits on textures is to test it (Unless someone has the figures).
Still there's nothing to stop you using this size if you want unless the map won't compile with it!