There are many factors governing the precise measurements for text pages and margins, but here are the basic terms you are going to come across.
What size should my book be?
There are no hard and fast rules other than it makes good sense to opt for a ‘popular’ size. I would recommended 12.9cm x 19.8cm (5.1″ x 7.8″). This is called B Format and is common and acceptable in both the US and UK. (this size has been traditionally used for ‘trade paperbacks’ in the US and ‘literary fiction’ in thr UK)
Don’t try to make these too small just in order to cram more text on a page. Again most printers will make recommendations, often specifying minimum sizes. The inside margins (or gutter) will commonly be larger on a paperback than the outside margins as a tight binding can make it difficult to open it flat. (those of you who habitually ‘break’ the spine when reading a paperback book can skip this advice)
Suggested margin sizes for B Format layout
These will work for most print suppliers and are what I use for my novels. I’ve given measurements in inches and mm. (alternatively, just find a book that looks right and measure the margins and page size – then copy)