By interval size, I assume you mean like 0 - 5, 6 - 11, etc.
Start with a number equal to or lower than your lowest number. I'd normally tell you to start with a "neat" number like the nearest 5, 20, 100, whatever... But 6 is kind of odd so that doesn't really matter. So you may as well start with your lowest number, which let's pretend is 8.
The easiest thing to do is put in the low end of the class and just keeping adding 6 until you go past your highest number. So let's say your highest number is 34. You want to go past 34. (Oh, layman's terms. The class is the group, like people ages 18 - 25 or something, lower limit being 18 and upper limit being 25.)
So you have a list thus:
The reason I recommend this is so that you don't have to think about what the upper number is. And makes it easier not to screw it up. It's easier to just keep adding 6 to each one and not worry about the upper number of the group.
Then go back and add the high end, which is the number just below the next lower limit. That is, what's right before 14? What's right before 20? Etc.
Then drop that last one. That was just there to make it easier to get the upper end of the last class.
This is really useful if you have decimal numbers. Like if your list includes stuff like 8.2, 10.6, etc. You want your upper limit to be a .9 number. It's much easier to list all the lowers limit first, then I can just go through and quickly put in 13.9, 19.9, etc. For the upper limits, without really thinking about it much:
8 - 13.9
14 - 19.9
Then of course you have to go back to your numbers to see how many you have for each class. Just go through your list and making a little tick mark next to the one it belongs to is easier. Then count the ticks. And make sure it adds back up to your total.
If for some reason this ends up looking weird, like you end up with lots of numbers in a couple of the classes and practically nothing in others, you can "shift" the whole thing down a couple of numbers. Like you could make it:
5 - 10
11 - 16
17 - 22
Etc. And see if that works out any better.