Change Text Box size?

Hi eatwellfit, of course you can.

.wpcf7-textarea{
    // put your css here
}

Here are the various classes of cf7

.wpcf7
    .wpcf7-form
    .wpcf7-form-control
    .wpcf7-form-control-wrap
    .wpcf7-text
    .wpcf7-email
    .wpcf7-textarea
    .wpcf7-submit

You can target each part of your form and style it any how you want. Read more from styling cf7

I’m still new to websites and only know a few basics.

Can I just copy and paste the above?

Where works i paste it? In my custom css stylesheet?

Sorry for being clueless ??

You will need to be skilled at using HTML and CSS to be able to do this. If that description does not fit you, you can learn all you need to know about both HTML and CSS at w3 schools

Also, the link I provided in my first comment contains all the steps you need to achieve what you want.