Ikea standard are 15" deep. Used to be 12" deep.

My old cabs are 12" deep. My cabinet maker is recommending 15". I'm worried about smacking my head!

As deep as your largest dinner plates require, if you’re putting than in cabinets.

If your lowers are only 24" deep I wouldn't go deeper than 12" deep for precisely your reason, also if someone climbs on the counter to reach a top shelf (as many children do, if this is the are in your home) there is less space for balancing. However some factors may make me change my mind.

How high will they be placed? - Standard is 54" high but if they will be higher and out of the way then deeper may be a good option

If your lower cabinets are say 30" deep then 15" deep cabinets may be nice to make them easier to reach and give you the extra storage.

Do you have a large island where most of your prep will be done? If so you likely will not be doing much at your outer counters and so there won't be much opportunity to hit your head on them.

These are the things I would consider when making that decision.

AFritzler , thanks! The lowers will be 25" deep. Yes, we will have a 4'x9' island for prep with a 2nd smaller sink and dishwasher. I will look at the plans to see how much clearance is between the counter and start of the uppers. I know I have my lowers and counter reaching a height of 38" off the floor (because we're tall).

I have 15" deep uppers on either side of my hood. Those couple of extra inches make a huge difference in what can fit. I use them mostly for serving pieces and wine glasses. On the other side of the kitchen I was only able to have 13" deep cabinets. I much prefer the 15" deep cabinets. They are over 25" deep bases. You won't smack your head.

15" is idea for storage but not for your prep area (had that in my last house) and I'm only 5'3".

If you can prep on your island that can work as long as the counter space you'll use is close to the stove. I hate it when the stove more than a step or two away.

Since you are taller I suggest specifying that your uppers be a bit higher than normal, make sure there will be enough space for any appliances that you will have on your counters. I'd suggest 56"(min)-60" off the floor.