How to increase the font size of one word only in the line?

<p class="call">To book a consultation click here or call <span class='bigger'>NOW</span></p>

with CSS

.bigger { font-size:200%; }

1. Decide why you want it to be bigger (the point is HTML is to describe semantics, not presentation, so you need to think about this sort of thing)
2. Write appropriate semantic markup
3. Apply CSS

Perhaps:

<p class="call">To book a consultation click here or call <em>now</em></p>

and in your stylesheet:

em {  /* or .call em, but you should aim for consistency in your styling */
    font-style: normal;
    font-size: 120%;
    text-transform: uppercase;
}