What are the regular kitchen island sizes?

It depends on if you are custom building, using stock base cabinets, and how much room you have to work with. For example my old home had plenty if room, but, my new kitchen is half its size. There are requirements for space surrounding the island. Good luck and stay safe!

https://www.laurysenkitchens.com/blog/dont-make-kitchen-island-design-mistakes/

Hi! there is no "standard" size. You build what you need. Here are some ideas:

https://www.familyhandyman.com/kitchen/the-12-best-diy-kitchen-islands/

There is no standard size as far as width and depth---you just design/buy what fits the kitchen best. But height can vary. The standard height of a standing/working island is 42 inches. 42 inches will accommodate a 30" seat-height barstool---but the height of your island should be at a level comfortable for you to stand and work, so adjust as you like.

Easiest method is to buy stock kitchen base cabinets. The size will be determined by your space available and the type of base cabinets used. Our island have 2 cabinets set back to back with a space between them and it is 3 sets of cabinets side to side. The dealers will sell you plywood panels to cover the sides and bridge the gap between cabinets. Ensure you figure in electrical receptacles for each end. Pre-run the wire to the island area.

They vary greatly so it is normally determined by the kitchen area it would occupy.

I would measure the size of your kitchen space to find the perfect size

The mistake most often made with kitchen islands is not enough space to move around the island. If you don't have at a minimum of 36" and preferably 42" on all sides of the island you will have more of an obstacle than a comfortable workspace. Consider instead a work space on wheels.