Yeah Santhol, you are partly correct here.
I do sometimes take help of my surroundings .... say, any average door has to be 7' high and 3' wide. Similarly, on site, I can make use of my arm ... have fixed on a perfect measurement of a 'foot' from my elbow to a point on my palm. Yes, definitely tiles, but only those which are 2' x 2' used universally in the server/data centre rooms.
And, oh yeah, the most important .... experience. The more you see, you have to have the attitude to absorb .... on estimations. This applies to not only 'sizes' but also 'costs' of items.
Obviously, it does not apply to electronics but predominantly metal / plastic hardware. Look at the equipment, do a 3-dimensional visual analysis, hit on the specific density of the material of construction, have an approx. idea of the raw material cost in kgs, apply your own conversion cost (that comes again by experience), put in a viable profit margin and ..... lo ..... you come up with a fairly good final price acceptable to a seller. As per my 'expert' bargaining-oriented wife, I get by 'on the spot' 90% of the times. That's pretty good by any standards, I know.
So, every time my wife goes shopping for ..... clothes, utensils, 100s of types of other goods, bags, fabrics.... you name it .... and shows me the 'contrabands' to check the prices ..... get it right MOST of the times.
So, she insists on my presence during her splurging sessions
, in the Delhi heat and dust and chill ... and ..... my poor salary.
But you need to know if it is a 12" tile or a 15" or 24" one you are looking at. The tiles on my floor are roughly 15.5 not 15 or 16"
Cheers