I always imagined some of the scenes from Hackers mixed with things like Reboot could give a good idea of what the Matrix can(though doesn't have to be) like. Just my suggestion though.
Also, my go-to rule for general descriptions, not just matrix or anything, is if it's something brand-new, a first time experience for the players, keep it to a small paragraph or a few lines. Remember to use the 5 senses, because if they're in VR or have simsense stuff, that can be important.
If it's familiar territory, keep it to a single line, just detailing any changes they notice to the landscape.