See my suggestion above to enclose the pattern with a container element and apply the css via that element.
Suppose you have a textarea field called 'Field 01'.
In the entry template you could have something like:
<div class="my-custom-field-size">[[Field 01] ]</div>
Then in the css tab you could add something like:
.my-custom-field-size textarea {
width: 300px;
}