Cannot style radio buttons

I want to change the default style of the radio button label and the text associated with each button but cannot find the right CSS selector.

The label is the default <p> selector but I want it bold as well, but cannot find a way to add a CSS class to the label – is this possible?

For the button text, Dev Tools tells me each element is the class wpcf7-list-item-label, but using this in child theme CSS:

.wpcf7-list-item-label {
font-size: 16 px !important;}

yields “Invalid property value” on the page for the 16px declaration. The other possibility for the element is ‘#top label span’ but that also gives “Invalid property value”. How can I change the default styles on this text?