<select name="questions[] " size="5" multiple>
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Just create a custom template for the block's view and append the form's name by "[]" to allow saving multiple answers in the array then you can add the word multiple to allow selection of multiple choices.
Here's an example:
You should definately check this example out to see how the select tag works when you give it the size attribute:http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_select_siz...