It sounds like you're in the advanced options, so below the "Add description" area there should be a resize option. If you don't see it, please make sure you have the latest Eye-Fi Center installed (3.4.24 on Windows, 3.4.28 on Mac, download at http://support.eye.fi/downloads/
You can also view the Gallery settings in your online Eye-Fi account at http://center.eye.fi/
. Choose the card on the left, click the gear icon to open the settings, and look for Gallery under Photos and click Edit. The Size option should be near the bottom.
If this didn't help, please add Gallery to Eye-Fi again and let us know if that helps.