This is another example of what Gunrunnerjohn referenced in another post, not having enough information. I'd never heard of this company so I did a couple google searches. First one took me to their site, but no mention of decals. Second search I typed in decals after the name and here's what I got... Is this the company you are referencing to:
http://islandmodelworks.com/Decals.html
If so, the answer is easy. These are not 28" and 30" scale. These are HO SCALE decals - half the size of O scale decals. The inch number refers to their scale height in HO scale. So item 8626 (24-inch high Long Island M hearld) is a quarter of an inch high in O scale.
Being an "027 scale" guy, I often can make use of HO scale decals, utilizing larger ones like from locomotives. That said, a lot of these, especially the lettering, would be on the small side, even for the 2400-series 11-inch Lionel streamliner passenger cars. Their HO 13-inch high Long Island lettering would be one-eighth of an inch high for O.