I usually run the Maxxis M6006
I have tried the 130 and 140 width. I couldnt find a 140 so I fit a 130 instead.
I wouldnt go smaller than 130 again... it looks crap and doesnt last
130 refers to width: does the other measurement matter, the aspect ratio? because mine is a 120 / 80, i want to go 130 or 140 wide, but also want to get a tire with x / 100, but cant find bigger than 130 / 90 or 140 / 90 neither are offered in x / 100.
If you get a 120/90x18 and a 120/80x18 same brand same thread pattern you will actually see that if fitted onto the same rim the /80's cross section is lower than the /90
Across different patterns and brands there is very little consistency. You'll find that one makes 120 is considerably narrower than another

Aggressive knobblies tend to be wider than 50/50's or stuff like Anakees of the same nominal width.
Kenda K270's are aleways narrow and look like knobbly pram wheels

In theory a 120/80x18 is supposed to be 120mm wide, have a cross section height of 80% of its width and is supposed to go onto an 18" rim. The last bit is about the only thing that's 100% reliable.
It gets really crappy once you get bike tires in imperial sizes (old standard & new standard) as the "fractions" of the tire width rarely represent fractions of inches

Correct me if I'm wrong:
2.75" tire translates to 80/90
3.00" to 90/90
3.25" (old) and 3.60" (new) to a 100/90
3.50" (old) and 4.10" (new) to a 110/90
4.00" and 4.25" (old) both replaced with 4.60" (new) looks just like a 120/90
5.00" (old) and 5.10" (new) put up as much bulk as a 130/90
There's also other rubbish like 4.50" & 4.75"
Some manufacturers still offer both old and new imperial standard tires

Rules:
On most bikes you can fit one size wider than OEM.
Check for enough clearance ALL AROUND the tire sides and top. You should have at least five millimetres on the sides and a centimetre between blocks and wheel cover / swingarm. Don't forget to check clearance to chain guards.
Check for clearance at full compression of suspension. Exhaust!

Measuring something before the tire is fitted and inflated does not help a lot. The rim width has influence on the width of the fitted tire.
At the end of the day it's your responsibility to make sure the tire fits.
A fitment centre will often fit whatever you tell them to. If you want them to take the tire back and give you another one that actually fits it might be a different story.
A wider tire of the same pattern and brand giving you better mileage than a narrower one due to it being wider is rubbish.
On the same rim the contact patch is actually slightly narrower.
The reason for the longer tire life of the wider tire is usually a harder compound as wider tires are designed for heavier, more powerful bikes.
The longer life comes obviously at the price of reduced cold & wet tar traction.
The wider tire often has advantages in sand.