If you want to change the size of this window or display the entire text there without scrolling down the text, add following css style code into your signup theme. Go to Merchant panel > Configuration > Design > Signup page theme click Edit theme button. If this theme is original, it will create new theme for editing.
Next, search file custom.css in theme editor and add this code there:
.TermsAndConditions .FormFieldInputContainer {width:350px; height:200px;}and change the width and height of the text box with Terms & Conditions to desired size.
.TermsAndConditions .FormFieldInput {width:350px; height:200px;}
.TermsAndConditions .TextBoxContainer {width:350px; height:200px;}
.c1_WrapperContainer .TermsAndConditions textarea.TextBox {height:400px;}
.c1_WrapperContainer .TermsAndConditions .TextBoxContainer {height:400px;}
For IE, add this line:
.TermsAndConditions .TextBoxContainer .TextBox{width:100%; height:200px;}
If you are using Compact theme, this formatting would looks like this: