Saddle height is the one thing that will be pretty much the same distance for you on most any bike you ride of the same genre of bike (road bike to road bike, mountain bike to mountain bike, cruiser to cruiser, etc).

What will affect you when sizing down on the same model of bike is that the frame stack will be lower and you will have more drop from the saddle when at it's correct height to the handlebars. To a lesser extent you might be cramped with your knees to close or your reach to the bars too close. Depends on the exact model of bike and how they scale the geometry for other sizes.

I'm 71" tall with a 34.5" inseam. I use to ride a huge 25" Schwinn from the 70's comfortably, then sized down to 60 and 59 cm bikes from those vintage days. I now ride a 56 cm Specialized Tarmac and wonder what a 54 cm Tarmac will have been like.

That would mean 21mm (≈.8") more bar drop for me that I could make up in spacers under the bar if I didn't like the drop, but the bars would probably look silly high on that bike. A 54 cm Specialized Roubaix with no spacers under the stem will probably give me the same saddle to bar drop that I have now on my 56 cm Tarmac with 15 mm of spacers under the stem.

So when you size down you might need to consider changing to a different model of bike if you can't deal with the increased bar drop for your position.

Welcome to BF!