Some time ago people were having trouble making boot USBs from sticks that had more than 4 gig capacity. I dont remember why. Thats why I put Ubuntu on a 2 gig stick and that works ok. There is nothing else
on the stick.
With that said Is that a problem still???
Also If I want to make a usb stick with 8 gigs and put 3 different distros on that one stick ,is that a problem when it goes to load
up??? Can the programs that make the boots from the iso put more than one on a single stick??

I never heard of the problem of creating a boot usb from a 4GB or larger flash drive. Never had a problem doing it.

No, not if it is done properly. Might be for someone unfamiliar with the process but it isn't difficult. That is, if you are talking about a Live CD on a usb. You won't likely get more than one, maybe two installs on 8GB with most major Linux distributions.

I have a flash drive with 10 different Linux systems in iso form that all boot. I created it and the grub.cfg file manually so if you want some software program to do it for you, maybe. Most of this software limits it to certain distributions from what I have read. You do know that many distributions will not boot from an iso directly even with Grub2?

There are basically three common ways to make a usb.

One is to use a live cd to usb tool that relies on using a FAT format (now ntfs is supported) in this method various improvements have been made. It is generally a fast way to use live media.

Two is a way to get an image of one or the other that you can copy to the usb on a bit by bit copy. Many exist even if they don't say hybrid image or such.

The very best way on modern large size usb's in my opinion is to use a normal install. Almost every linux now see's the usb as a hard drive. (subject to system support) It allows you to do almost everything you can do on a real internal hard drive.

The problem was with creating a persistent storage area, which was limited to 4GB. That problem is solved by using a casper-rw partition as opposed to a casper-rw block file.
If you are not creating persistance on the flash drive, then that is completely irrelevant to you.

You can do what yancek does, and create the grub.cfg file manually
INSTRUCTIONS ARE HERE

Or you can install the Multisystem program and use that to create your USB. It works fine. I use it.
INSTRUCTIONS ARE HERE

Or you can download and burn the Rescatux/SuperGrub2 CD.
That one is my favorite. It will easily load your .iso files either from a flash drive or from the hard drive.
INSTRUCTIONS ARE HERE

There should be no difference installing to an 8GB USB flash drive or an 8GB HDD. Consider, however, that many popular distros require at least 4GB of available space. I've got a 4GB flash drive with four different Puppy Linux variants on it - I installed them manually.