Moka pots are great for many reasons. One of those reasons is that you can get good coffee out of almost any reasonable grind size. My recommendation would be to play around with it, taking notes on grind variables and resulting taste profiles.

If your grinder is a blade grinder, I would recommend to grind to just as the grounds start to stick to the bottom. For me, that's about 20± seconds for the 4 mark. I only usually use a blade grinder at work, and only sometimes, as I prefer a burr grinder. Don't grind finer than espresso, or you risk a pipe bomb type of explosion. Moka pots can be subjected to massive amounts of pressure during the brewing process, and too fine of a grind can clog the release, essentially creating a bomb (one of the scariest days of my life).

Here's what I do and it works really really well. Gives perfect coffee every time, but your milage may vary so don't be afraid to experiment.

Grind to filter coffe size or maybe a bit finer. Don't go to espresso level or it'll just clog the brewer and burn the coffee. It is generally not super sensitive to grind size so don't worry too much. If in doubt go corser.

Always, always weigh the coffe! I found that they are actually quite sensitive to the amount of coffe you put in. Ever since a started weighing the beans I've never had any issues with it. If you cannot get a nice consistent flow through the entire brew and it begins to sputter too early you most likely filled it too much. Rather than fiddle with the grind size, you should instead fill it a little less next time. I use 12-13g for the 2 cup, and 28-30g for the 6 cup version. If I go outside that range it won't brew nicely. It's that sensitive.

Finally use hot water. This first of speed up the brew but also makes it easier to control the pressure inside. So pour hot/freshly boiled water in, assemble, and put on a low to medium heat. When the coffee starts to come through turn it down to low heat to get a nice, slow and consistent flow. For the 2 cup version it's about 15-30 seconds and for the big 6 cup version it's about 40-90 seconds. I can't remember exactly so don't worry too much as long as it's consistent and nice and slow.

Stir and pour the coffee immediately. Otherwise it'll slowly burn and turn bitter in the hot brewer.

With this method i always get amazing coffee out of this little machine and i low it so much.

I hope this was helpful.

Edit: I see from other comments that you use a blade grinder. This is fine, and I've used one for years when I was a student and it worked just fine as well. You can grind pretty fine with that for the mokka pot, but again, if on doubt go corser. More importantly is to avoid clumps. Blade grinders tends to make clumps so try to stir them out a bit with a needle/scewer or a very thin fork. Clumps cause channeling and easily makes it sputter and all the water goes through too fast since it cannot build the pressure needed to make a consistent flow. On the other hand if it sputters and goes really slow then it's clogged instead.

They take a pretty fine grind, close to espresso. How long you grind depends on too many factors for us to answer here. You just grind until you have enough :-)

Dang, we are working on a chart to help with basically this question, but not enough input yet. I assume you got the moka pot? We only have one setting information there and it's for the fellow grinder so that should be between medium fine and medium grind size (550-625 microns)

I'll be taking any answers you get and bringing them over to the chart as it's very empty right now.

dont go espresso fine. moka requires filter-range grind. your exact grind and prep can have a real impact on your brew so doing some experimenting with grind size may be rewarding. you may be able to grind finer (within the filter range) for moka. the main issue is that moka naturally carries fines up to the collection chamber, so i recommend starting with a medium filter grind to keep the brew quick and free-flowing. it can also help immensely to sandwich an aeropress filter above the basket to reduce the fines.

One of the store bought brands that's suggested for the Moka Pot is Cafe Bustelo (yellow pack), so anything that's got a similar grind should be A-OK.

https://www.cafebustelo.com/en/coffee/recipes/cafecito

https://wolfewithane.com/secrets-of-the-moka-pot

"The grind of Cafe Bustelo is a bit coarser than an true espresso grind, but finer than standard grind: exactly what’s needed for a moka pot. The true expresso grind is too fine and can seep through the pores, clog the filter, cause incorrect pressure, result in muddy coffee and leave deposits that taste rancid."

between filter and espresso. similar to aeropress. I tend to grind at what would be considered the finer end of filter/pour over and the coarser end of aeropress.