css is stupid u have to say px
and there is a semi-colon after

I copied over the custom.sample.css to custom.css and added to the bottom of the file;

.clock .time , .seconds{
font-size: 200
}

but saw no change, rebooted the server, no change.

so in custom.css

you would put ( notice the spaces between elements)

edited, to add commas

.clock  .time , .seconds{
    font-size: ???
}

this is font-size: 80px;