Changing jQuery UI Button size?

If it’s styling ui-button-text with font-size directly, you can override it at a higher level by applying !important. Such as:

.small-button {
   font-size: .8em !important;
}

EDIT: try setting a CSS style directly on .ui-button-text to inherit:

.ui-button-text {
   font-size: inherit !important;
} 

This should also make the !important on the .small-button irrelevant.