Bought an 83 year old house with old heat registers that are in need of replacement. I removed all of them and after measuring the openings, I'm realizing that the duct openings all vary in size by .5-1.5 inch.

Generally, most of them fall within the 14 x 7 range (duct opening size) with variations of 1/2 to 1.5 inch. The previous registers were all the same size which makes me think I can repurchase new ones in the same size as well.

The question is, do I go on the larger end of the range or smaller?

https://www.homedepot.com/p/Everbilt-14-in-x-6-in-2-Way-Steel-Wall-Ceiling-Register-in-White-E102M14X06/300539285?source=shoppingads&locale=en-US

I'm considering these from home depot.

For instance, an opening that is measuring 14.5 x 6.5 do I purchase 14 x 6? ( I think the easy answer there is yes) but there is an opening in the same room measuring 13 x 6.5 - do I go down a size and buy 12 x 6 registers?

Or should I purchase 12 x 6 for all the registers as the lowest common denominator? (this is what I'm thinking is the right answer now that I'm typing this out). Some will be snug in the vent and others will have "space" around the vent and register. This is OK right?

Thank you