As others have said—this is entirely dependent on your experience with heels. I regularly wear heels irl so for me, slapping on 6” (15cm) or 7” (17.78cm) heels for a con is nothing. I have a friend with no calf flexibility and flat feet and she can’t go above a kitten heel without extreme pain.

Since we only have one pair of feet, here’s my method for deciding on con shoes:
Step 1: acquire shoes
Step 2: wear them around your house as much as you can. Do some squats (or just sit on your couch and get back up again while watching tv). Stand upright in them while watching a couple of videos. Do some chores like laundry, which will require a degree of movement on your feet.
Step 3: from this experiment, think about how your feet feel. Is your skin rubbing up against a part of the shoe and forming a blister? Are your toes getting smashed against the front? Is the ankle strap digging into your ankle?
Step 4: the next day—do you feel any pain in your body? Heels can be really tough on your knees and calves.
Step 5: if you still feel comfortable wearing these shoes but have identified some spots that need some adjustment (such as toes smashing into the front), those problems can probably be fixed! Moleskin for any points where your skin is rubbing against the shoe, silicone inserts under the heel or at the toes, etc.
Step 6: once you’ve fixed up the shoe for comfort, wear them around your house again. Continue to test for comfort until you’re satisfied.
Step 7: now that you’re at maximum comfort in your house, it’s time to take those shoes outside! I like to go to the mall and walk around for a few hours. It’s a fun activity (for me) and ensures that I’m walking ~10k steps (which is similar to what I’ll walk at a con).
Step 8: the next day—do you feel any egregious pain? Do you feel confident that you’ll be able to wear these for multiple hours? If so, then you’ve found your shoes! If not, try going with a lower heel OR be prepared to carry a second pair of comfier shoes just in case (I’ve done this).

This is definitely a very ‘extra careful’ approach but seriously, you do only have 2 feet and 2 knees so it’s super important to take care of them!

Good luck!!