The CSS tag you would want to use is cf-label. The only tricky part is the specificity because the built-in styling has a high specificity that overrides simple CSS.

Rather than fighting specificity, you could take the "scorched earth" approach and use !important

For example,

/* Does not work. Overridden by built-in styling */
.cf-label {
  width:200px;
}

/* Overrides most other styling */
.cf-label {
  width:200px !important;
}

The "correct" way would be to research the specificity hierarchy chart to get a feel for how browsers decide which styling to use when you have multiple things affecting the same element, but sometimes !important is the only option because the built-in styles are too specific to override with selectors alone.