Forms: add text size control to concent block

Concent block is small by default, without way to adjust its text size. Here's comparison with regular checkbox field:

Let's add text size control to the block, and default it to "Small":

It should be possible to make the field same size as any other field, if user so wants.

Start by locating the Forms Concent block implementation and comparing its available controls with the regular checkbox field shown in the issue. Done means the block exposes a text-size control defaulting to Small and allows the rendered field to match the size of other fields.

Written by the indexing model from the issue text.