PDF/X-4 is the standard but your printer may have different requirements.
Best to ask them.
should I tick the crop and bleed boxes?
Did you use bleed? If you did, then you absolutely need to include it, and the crop marks (bleed marks are superfluous). Even without bleed, crop marks are a good idea.
I recall a conversation somewhere about the marks somewhere. Many
printers don't want them and will remove them and add their own. A
properly constructed PDF will have the correct bleed information in it
and the printer can easily add the crop marks where they want them.
Indeed, most impostion software that I know about will discard the marks we supply, but I add them anyway. It's a good cue for anyone looking at the file on screen, and it's not a hassle for the printer the way sending spreads would be.
I have selected crop and bleed (3mm).
The designer who set up my Indesign template sent me a 28mb file when he saved it to PDF. I have just saved mine in the press ready format and its only 7mb - anything I might be missing?
Was the previous PDF 28 mb, or the ID file?
There's no real correlation between the size of a .indd and the resulting PDF. It depends on what's in the file and the settings you use to make the PDF. If you've followed the printer's directions, you'll be fine.