A) Which version of Illustrator do you have?
B) How do you generate the PDF?
Regardless of the answer to the above:
– The “save as” is the most straightforward way to do it.
– I don’t know any way to accidentally shrink the document. Illustrator saves the PDF document at the size it is. I would check that you have the document settings how you think they are (inches/cm/points/pixels).
– The double crop marks are probably “Japanese” style cropmarks, select “roman” instead.