Oh okay, I thought you wanted to change the width.
The font sizes are mostly inherent from the theme, but you can use these custom CSS codes to change their sizes:
/* Legend size */
div.asp_w.asp_sb.searchsettings fieldset legend {
font-size: 16px !important;
}
/* Checkbox option size */
.asp_option_label {
font-size: 16px !important;
}