The usb 3.0 disk connected to my Macbook Pro Retina 13" keeps telling me "Your startup disk is almost full...delete some files. What size disk should I get to hold more?

This warning is about your internal drive, not your USB external drive. You need to remove files from your internal drive before it becomes too full and is corrupted. Be sure to start doing a backup.

How to Free Up Space on The Hard Drive

1. You can remove data from your Home folder except for the /Home/Library/ folder.
2. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
3. Also, see Freeing space on your Mac OS X startup disk.
4. See Where did my Disk Space go?.
5. Be sure to Empty the Trash to recover the space.
6. Replace the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.
7. Use OmniDiskSweeper or GrandPerspective to search your drive for large files and where they are located.