I laid a tape measure on the floor and put my foot on top to get my measurements, is this the proper method? From 2nd long toe to heel, I am about 26.5cm in length (do I measure to the back of my heel that actually touches the floor, or all the way up the curve of the heel that goes a few more mms?)
And about 11cm in width where my foot is widest at ball of foot to the other side.
Is this a standard measurement for a US size 10?
Hard to tell from your description, but probably not.
I recommend to find a wall and use this as the starting point for your measuring tape. A wall in the bathroom is probably the best option, since you can fix the measuring tape to the floor with Scotch tape. Put your heel against the wall, and check the length to your longest toe.
Also, measure in Millimeter and don't round up or down to half a Centimeter.
To give you an idea why this is important: If your foot length were truely 265 mm, your reference size(s) would be a UK 8 and US 9. With that foot length, the average ball width would be something close to 102 mm. 110 mm would be very (very) wide. This is not a D width. I'd expect something more like a EE or EEE.
So, my guess is you probably mismeasured. It's important to stand upright while measuring, since kneeling will decrease your foot length, and affect any information you'll get.
Hope this helps.