Use selective sync
Dropbox syncs all your files to your computer. That's no big deal if you only have a couple gigabytes' worth in Dropbox, but things can get hairy if you have a hundred or more. Dropbox syncing does consider storage limitations--it automatically it syncs your smallest files first and continues until you run out of space. But that may still leave you without the presentation you need for your afternoon meeting.
The best way to save space and make sure you absolutely have the files you need is to use Selective Sync. This feature lets you designate per computer which folders to exclude from syncing. Not only does it help you manage your storage space, it's also a great way to make sure only appropriate files are synced to your home and work computers, respectively.
To enable Selective Sync from Windows (Vista, 7, or 8), click the Dropbox icon in the system tray. Next, click the gear icon and choose Preferences. Then select Account > Selective Sync. A window will appear showing all your top-level folders with a checkbox next to each. Uncheck the box of each folder you want to exclude from syncing and click OK. Those folders will be removed from the hard drive of that computer, but will still be available on your other Dropbox-connected devices and through the website.
Sometimes it's not your storage but your bandwidth that's at a premium. In those cases, syncing a lot of files, no matter what the size, can cause a bottleneck in your data flow. The easiest way to clear it is to temporarily pause syncing.
To do this, click the Dropbox icon from the Windows system tray. Click the gear icon and select Pause syncing. You'll see a pause icon over the Dropbox menu to show syncing has stopped. To start it again, follow the same steps and select Resume syncing.
Even if you won't want to completely put the brakes on syncing, you can still slow it down. By default, Dropbox throttles itself to 75 percent of your maximum upload speed and downloads at the fastest speed available. But if you're still browsing at a snail's pace, you can customize these settings to improve performance.
Click the Dropbox icon from the Windows system tray and click the gear icon. Select Preferences > Bandwidth. A window will open with fields for your upload and download rates. Click the Limit to radio button next to each and enter your preferred rates in each field and click apply. You may have to experiment with different rates to find the ones that work best for you.