Change font-size or colour of a piece of text in a content block
create a clone of the content block and add:
'plugins': ['fontfamily','fontsize','fontcolor','concrete5', 'my_own_plugin']
when initializing your redactor element (form.php)
This is standard included in my User_Info block, but it might be a bit overweight for your purpose.
at least in 5.7.4 you can select those plugins