First of all, try this for all select fields, it might be overkill for your needs. Drag an HTML block onto your logo & input the following CSS:
<style type="text/css">
select .mktFormSelect{padding-bottom:50px;}
</style>
If that doesn't do it for you, You'll need to use Google Chrome to right click & "Inspect Element" on the form field you're looking to target. Then you'll see an id & name in that
select.
<select class="mktFormSelect mktFReq" name="picklist1" id="
picklist1" size="3" multiple="multiple" tabindex="11">.....
In this case, if I want to change the style of the select field with the name of picklist1
<style type="text/css">
select #
picklist1{padding-bottom:50px;}
</style>
Please note the semantics at play here. The CLASS in the first example has a . before the name. The ID in the 2nd example uses a # before the name. This is true for all CSS, if you're describing a class use a . or an id use a #.
If you aren't comfortable with the difference between class & id
here's a little tutorial.
Let me know if that works out for you.