Welcome to the hobby and to the site.
50g as mentioned is a pretty good size.
However, if you are like most hobbyists, it won't be long before you regret going that small if you can afford larger.
If you have room for more tanks to add in the future then it's not a problem as I went from one system up to 10 systems within about 2 1/2 yrs.
If not, you almost certainly would be better to go bigger now.
The larger you can make your sump, the more total water in the system, and the more forgiving your set up will be if something gets out of wack, giving you a bit more time to straighten things out before losses set in.
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