It's interesting that you're seeing a width of 7px for your input textbox. The RadGrid skins should be setting this width to 20px by default. I bet you may have a general CSS rule (like a rule applied to all input controls) that's "cascading" on to your RadGrid and causing the unusually narrow pager textbox. Try focusing your site CSS and that should clear things up.
If that's not an option, you can also use the CSS !important directive to change the input's width, like this:
width: 20px !important;
Let me know if that helps.