Pam,
Since I don't know exactly what you're doing I don't know if you're doing it incorrectly. If you are trying to create a new Yahoo ID, usually the best thing to do is to pick a name and add some numbers on the end which makes it unique pretty easily. Like Pam2346 or something.
If you are trying to create a new Yahoo ID for an email address that you already have a Yahoo ID for, then I think you will have a problem because it's seeing that there's already a Yahoo ID for that email address.
If you're just concerned about the name that shows up with the posts, you should be able to go to the group page (or the Yahoo group page for any group you're in) and click on My Account up at the top. Click the "Edit" button to the right of Address/Contact Information, and it will let you change your name.
However, I'm not sure that's where it gets your name. I think that really comes from your email program that you use for whatever kind of email you have. When you set up your email on your computer, there is a place where you get to determine the name that's sent with your email. This would be the same whether you send it to a Yahoo group or anyone else. There are very many email programs and I have no idea what kind you have, but this is true whether you have an on-line email account or one on your computer.
As far as the forum being in small font, I haven't run into anyone before with that problem. I'm going to guess that you need to go into your settings for your internet browser (for example, Internet Explorer) and change the font that's displayed there). I would think you might have a similar problem with other web pages you look at?
Kitty