Been asking this same question myself and searching for a Fix on here and VB.com.
https://vborg.vbsupport.ru/showthread.php?t=299841
Still Unsolved
Tried styles & template --> Style Manager -> edit template -> header ,
One click on header only shows..
header:
This template has not been customized in this style or any of its parent styles.
Double-click the template name or click the 'Customize' button to add a customized version of this template to the current style and its child sets.
Last edited March 22 2013 at 19:13 by vBulletin Solutions
..............................
Double click brings up the code with 363 commands, Yet i see nothing to adjust the size of the header Logo.
Where and What should we be doing to accomplish this, I have seen several members asking the same question on Here and VB.com and have yet to see anyone post about achieving it.