There is nothing in AcyMailing to customize the size of the inserted dynamic texts, the span with the class "acym_dynamic" will disappear in the sent emails.
There is a way to accomplish what you are looking for by directly editing the HTML of the block but it is a bit complicated.
Select the block that holds your text and click on "Edit html".
For example when you insert a tag to use the subscribers first name find this:
<span class="acym_dynamic mceNonEditable" contenteditable="false" data-dynamic="{subscriber:name|part:first|ucfirst}">Alex<em class="acym_remove_dynamic acymicon-close"></em></span>
and wrap it in another span like this:
<span style="font-size:20px;"><span class="acym_dynamic mceNonEditable" contenteditable="false" data-dynamic="{subscriber:name|part:first|ucfirst}">Alex<em class="acym_remove_dynamic acymicon-close"></em></span></span>
