Land Size

I am so beyond new but i need some help if anyone is willing to.

How do i cut land to: 16384m2 island

And how do i even come up with the answer to how big the land is, a friend told me the land size would be 128m by 128m... But can someone tell me how did he get that answer. I know it seems so beyond new and i tried calculating it but i still dont get it

A "standard" 512 sqm parcel is exactly 16 x 32 meters (16 x 32 = 512).
To get/see the boundaries, use prims and line them up precisely (set up the grid > build menu). A SIM is 256 x 256 (256 x 256 = 65536) usually, and square... Now, it can complicated if you slice in a "non standard way". With those figures I gave you, you should be able to understand it and cut it the way you want. Your 1/4 SIM doesn't have to be 128 x 128 for instance, it could be 64 X 256... etc...

Go to view > property lines... it will show you some boundaries, then right click on the ground > edit terrain.... click + drag and then cut, or join etc...

If you wanted to get a perfect 128 x 128 or 64 X 256 etc... , it would be best to line up the prims on the ground, of course, measure the prims..., then you can click + drag and slice...
Start at the edge or one corner of your/the parcel you want to cut, it has to be precise, that' s why you want to use the grid to line up faster.

Once you get used to it, you don't even need to use the prims anymore as a guide, you can do it by eye.