What is the size affordable for the USB boot flash?
The Ubuntu distribution easily fits in a 8G flash -- I mean an install not a LiveUSB.
If you are looking for a small LiveUSB, the PuppyLinux will fit in a very small footprint (about 256MB).
Finally, if you have a sharp target, you can head straight to the GParted Live distribution.
GParted Live is a small bootable GNU/Linux distribution for x86 based computers.
It enables you to use all the features of the latest versions of the GParted application.
GParted Live can be installed on CD, USB, PXE server, and Hard Disk then run on an x86 machine.
GParted Live includes additional packages, such as:
mlterm - Multilingual terminal emulator (provides access to command line)
partimage - Backup partitions into a compressed image file
testdisk - Data recovery tool that can help recover lost partitions
nano - Text editor
vim-tiny - Enhanced vi text editor
parted - Partition table editor (command line based)
fdisk - MSDOS partition table editor (command line based)
gdisk - GPT partition table editor (command line based)