The largest you can find. Gitty has one called jumbo pyramid that I use.
I usually use a finishing nail (4d).
Most six stringers have a zero fret the same size as the other frets, to get the lowest action possible.
I have used the same size as the others and a jumbo fret. Same size will get you a lower action. A jumbo fret is easier to do. Less fret leveling required to make it work.
The annoying thing about Zero Frets is having to make a string guide for all the strings.
Which is why I use a Brass or Steel bolt for a nut. Has built in guides. Just have to cut a channel for the bolt.
I use the same size as the others ,
Something i've been meaning to post here for a while now and kept forgetting to. Since i started zero fretting it has been my regular method generally using medium/ medium wire for all frets, i get a very good low action, but, as i have a damaged left hand and struggle to fret with any more than 1 finger, at the most 2, i tend to mainly use a slide and the medium/ medium combination is tough to use unless i raise the bridge to a point where fretting any where above the 5th fret is very sharp. To get around this, and keep a good low action for others to use I got some 4mm aluminium tube from a hobby supplier, cut it to the length of my zero fret, then sanded it until it was a "c" section, then slide it over my zero fret under the strings and it raises the strings around 1mm at the nut, it works well and takes seconds to add and remove plus a quick tuning tweak
They are traditionally same size as rest, if you use search function of this forum you'll come across a bevy of past experiences from members FYI, good luck.
Most of these gits don't have a trussrod and most that do have one have a non-adjustable type. So it's really better to have a zero fret that's a little bigger to lessen the chances of fret buzz. Especially after the wood ages and everything moves a bit since most of these gits use new wood instead of aged/seasoned.