Changing the form height would take more effort than the form width, you'll need to change the padding between fields.
To change the form width, search for this:
#form_container
{
background:#fff;
border:1px solid #ccc;
margin:0 auto;
text-align:left;
width:640px;
}
Change the width above.
You will also need to change the width for #top and #bottom, which you should see it below the code above.
To change the font size of your field labels, search for this:
#main_body label.description
{
border:none;
color:#222;
display:block;
font-size:95%;
font-weight:700;
line-height:150%;
padding:0 0 1px;
}
Adjust the font-size to suit your need.
To change the height of the fields, search for this:
#main_body form li
{
display:block;
margin:0;
padding:4px 5px 2px 9px;
position:relative;
}
Play around with the padding values to get the best result.
To change the width of the fields, it depends on the size you defined from the admin panel.
There are three sizes: small, medium and large.
So for example, if you have an input with a medium size, you will need to adjust this code:
#main_body input.medium
{
width:50%;
}
If you have a large input, adjust this one:
#main_body input.large
{
width:99%;
}
Are you using Firefox?
I suggest to install Firefox and then install Firebug plugin.
Firebug is a great tool for this purpose. You can simply click a form field and it will show you the respective CSS code which applied for that field.
Try to check it here: http://getfirebug.com/
Let me know if you need further help.