Hi
Its here
First Solution
while creating contact form the default is something like this
<p>email
[email your-email] </p>
<p>subject
[text your-subject] </p>
Now add text box size to it like this [email your-email 24/] </p>
Note 24/ is size, you can change it according to need, it will set the width of text box.
Now move to text area
[textarea your-message 19×3]
Note- 19×3 means
19 columns and 3 rows
rows will set the height of text box and columns will set the width of text box.
Second Solution
By using css,
open your default theme’s css file which is style.css
and append these line any where
.wpcf7-text { width:200px !important; }
.wpcf7-textarea { width:200px !important; }
the first one will set the width of text box and second one of textarea, now please adjust the width according to need.
!important; means any other style will be ignored or overwrite.
Hope it will help you.