Size of PDF

You're just talking about the view size and not the actual file size, correct?

On your end for the view size, you can go to Edit>Preferences>Page display>Default layout and zoom and set the Zoom settings to whatever you want. Keep in mind that this will effect ALL pdf files that you open with the exception of a pdf that the creator has made the settings to that the file opens at a certain percentage. If that's the case, your preferences won't matter.

When you email it to someone, they would have to change their preferences as well. You can't do anything on your end to change that if you only have Acrobat Reader. If you have the full paid-for version of Adobe Acrobat, you can set the open percentage in File>Properties which will work for you and the people you send it to.