The matter of belts depends entirely upon your attire. If you wear jeans every day, buy a gun belt to match. Same for khakis. If you wear professional attire, then get a dress gun belt from Ken Null or the nice dress belt from Galco. You want a belt with a stiffener inside to support the weight of any firearm. Most people will have a gun belt for each of the wardrobe types as no one wears the same clothes every day. Each maker has unique rules for measurements. That said, there is ONE common factor: The "size" of the belt in inches will be to the middle hole, and most have 5 holes. Ken Null normally has 3, with the size being to the middle, and the design of the buckle hides the other two, which makes for a really cool appearance. Ken will, upon request, add length and additional holes, if you want.
The one style of belt I cannot recommend for anything but range use is the canvas type of "instructor belt" (Wilderness is one brand, I think - there are others) so popular with shooting school instructor people. Those, along will photo journalist or similar vests scream out "I am carrying a gun." Don't overlook a pocket holster for the Shield as it is certainly suitable for pocket carry as long as you don't wear your pants so tight the pistol "prints." If you are serious about wearing 24x7, then you will have to adjust your wardrobe to your pistols. If you carry inside the pants, buy your pants one size bigger. If you carry inside the pants, be sure to measure your belt size WITH the pistol in place. Loose clothing is more stylish these days (Saturday Night Fever and "disco pants" went out 35 or more years ago).
Taking accurate measurements for a real gun belt is always depressing because the tape measure doesn't lie. And, seemingly no one wants to admit they wear bigger waist sizes than when younger. When in college, I worked in a men's clothing store, and men would come in when Levis were on sale for under $10.00 (anyone remember those days?), or to be measured for a tuxedo rental for a wedding. If the man said 34 waist, it would measure 38, if he said 36, it would measure 42, etc. The amazing thing is that somehow they would stubbornly fit into the Levis (the fastener was under the belly, which hung way over), and you never wanted to stand in front as the metal stud fastener might launch off and injury might result.
Properly fitting clothes fit better, and you have to buy bigger to also wear a pistol. Buy accordingly.