There's no way to do this with the way that Content Areas work. Custom content areas in templates are just placeholders. There's no point at which you can script against them after you've created your template-based email.
If you forego adding
<custom type="content" name="content1"> placeholders to your template and just use named Content Areas, you'll have more opportunity to control what is displayed. For example:
set @content = ContentAreaByName("My Contents\Content1")
if length(@content) > 10 then
outputline(concat("<br>oops, too long"))
/* or alternatively */
/* raiseerror("content too long, skipping email",0) */
This method is good for controlling the content, but it's a more technical way to build your emails. The skills of those deploying the emails is usually the deciding factor.