Hi Alhazred,
There's two ways of doing this, first one is the simplest but not permanent (meaning you would loose the edit every single time you would refresh the imageset in acp):
Go to acp >> styles tab >> imagesets
and set the sizes to the sizes you would like
Then purge the cache in general tab and maybe clear your browser if you don t see the changes
Second makes it permanent:
Go to black_pearl/imageset (where you probably have allready replaced the logo) to edit imageset.cfg
Find:
img_site_logo = sitelogo_small.png*150*150
Replace it with:
img_site_logo = sitelogo_small.png*150*300
Refresh the imageset in acp, maybe even purge cache(this never hurts

) and if needed also clear the browser
It is your choice wich way to use.
Goodluck,
.. Joshua203