I'd say the max-height makes some sense. It makes sure the top bar doesn't grow too much. On the other hand, it's true that it is an arbitrary value.
The height: auto !important is the one to blame, along with width: auto. These two are telling the image to take as much as space as possible, while keeping the ratio.
I wouldn't just remove the !important from the height because the selector applies to all images so the impact will exceed the logo and apply that change to all images.
The options I'd recommend are:
A) Changing the CSS of the file. This is a pretty clean approach as long as you take note of the changes. In fact, this means appending (to the end of the file) the following code to the qa-theme/SnowFlat/qa-styles.css file:
.qa-logo img {
height: 10px !important;
width: 50px;
}
Note making changes to the CSS will force you to apply/merge those changes whenever a new Q2A release is published. Also, changing the width, might be optional, of course.
B) Applying a similar code to the one above to the Custom HTML in <head> section of every page setting in admin/layout section. This approach will not force you to have to add the code again after a Q2A upgrade. However, you will have to check after te upgrade if things still look good or if the code is not compatible with the upgrade.
Following the previous example, the code to add in the textarea would be:
<style>
.qa-logo img {
height: 10px !important;
width: 50px;
}
</style>
