I think the site is being a little disingenuous so as to prove its point.
It uses the example with the volume of a solid gold wreath being 51.8 cc. The volume of wreath adulterated with 30% silver would be 64.8cc, a difference of 13cc or 25%, which I think would be reasonably easy to measure...
That is correct and deliberate. I read the poster's question "When I was reading this page to understand Archimedes' 'story'" and answered appropriately by giving the story.
The poster is not asking how it would be done today but is trying to understand what Archimedes did or could have done...
You may be interested in https://thestouracademytrust.org.uk/wp-content/uploads/2020/05/Archimedes-Story.pdf for a description of what apparently occurred - Archimedes realized he could measure the volume of an object by measuring the volume of water it displaced. I don't know how accurate it...
Go for the biggest wheel possible.
It gives a smoother ride and the scooter will not stop or skid when it hits a pebble and throw you off.
Differences in friction and drag are so small in context as to be irrelevant.