radNumericTextbox has built-in style and you won't overwrite your style to them in normal way, so for add your style first addClass to your radgrid and then use:
.yourClass input[type=text] {
width:100% !important;
}
also you can use firebug or inspector to find orginal radgrid numeric class and then overwrite the style with the !important like so:
.RadGrid .RadInput{
width:100% !important;
}