Displaying image size (other than full)

Hello,
I'd like to display image size other than the full size – including the stock "thumbnail"/"medium"/"large, or my own custom sizes. I believe I've setup the call properly in the template, however, in each case I always just get the full image – Am I missing something?

Here is what I'm using in the template:

to try and get the medium size:
<?php echo(types_render_field("pb-image", array("size"=>"medium", "url"=>"true"))); ?>

or to get my custom size:
<?php echo(types_render_field("pb-image", array("size"=>"thumb-tall", "url"=>"true"))); ?>

neither has worked, only the full image displays…

Thank you for your help.