I can only offer a partial response from personal experience, but I believe it can still be helpful.
The largest seem to be those from Redoubt Miniatures--quite large indeed-possibly even fairly being described as "32's."
Copplestone are only somewhat smaller, but requiring slotta bases which, if used, make them much taller than the other brands.
Bluemoon offers at least two sets of very useful figures--including an excellent selection of unarmed "Honest Johns" to share the streets with the gangsters. These are very close to Copplestones but with some deliberate variations in figure height and build between some figures.
Bob Murch's Pulp Figures are pretty much "true" 28's.
Lon Weiss' Brigade Miniatures are the smallest of all these from my personal collection, but not at all to be thought of as somehow "too small."
I do not own any figures from Great Escape, so cannot comment.
The newest range of figures are the "We Only Kill Each Other" (WOKEO) sculpted by Mr. Alan Squires of the UK on commission by Frank McGovern who produces Pulp Miniatures here in the US. These were specifically designed to be
"true 28's" but a few are somewhat slighter. At the risk of splitting hairs, these might be placed between Pulp Figure's and Brigade.
All this said, I can hardly think of a "Gangster Game" that must field "units" of figures representing companies/battalions/regiments! They are by their nature truly 1:1 games, and as such any differences in figure height/build are all well within the normal range of humanity.
That is, I encourage you not to let the differences in size concern you. By buying at least some figures from two or more--ideally all--of these vendors you can guarantee the most visually appealing collection, and all are worthy sculpts.
Honesty in advertising requires me to emphasize that I am not an altogether neutral observer in this as the publisher of the rules "We Only Kill Each Other" ( link ) for whom the eponymous WOKEO range was developed ( pulpminiatures.com ).
Good luck to you, Sir!
TVAG