NOTE - all version of Android are slightly different, so your screens might look a little different and your options might be in a slightly different place.
|Go into the Gmail app, press your menu button, then 'more' then 'settings' then click on the account you want to change.|
|Scroll down until you see the 'Sync inboxes and labels' button|
|Using this screen you can click to set the amount of mail you want to sync for each part of your Gmail account. I recommend just changing the top option to keep things simple|
|Here's what the box for setting the number of days to sync looks like.|