There are three options:
1. SD card (assuming you have a new 13″.) They’re available at up to 64 GB, obviously still under your 250 GB requirement. There are USB flash drives at up to 256 GB, but they’re insanely expensive ($700+,) 128 GB is slightly better at ~$300. I lump SD and USB together in the ‘ultra-small, but expensive for the capacity’ grouping.
2. 1.8″ hard drive. These are the same hard drives that are used in the iPod Classic. More expensive than physically-larger, at much lower capacities, but still way cheaper than an SD card or USB drive of equal size. Toshiba *JUST* released a 250 GB model, but it isn’t shipping in any devices yet.
3. 2.5″ hard drive. Bigger than a 1.8″, but still not too big (and USB-powered.) Available up to 1 TB. This really is your best bet. They’re not too large, yet they are available in high capacities, for not too much money.