There is no way to do exactly what you are specifying in Preview.app without doing a bit of math.
Holding down the shift key while using the select tool will force the select box to retain itself in square dimensions, while holding down the option key will open the select box from the center of where you begin your selection, while holding down both shift and option keys will implement both these features simultaneously. After creating the size of the select box that you want, you can use command-K to crop. Using the Adjust Size function in the Tools menu retains the aspect ratio by default.
Not what you asked for, but when using the select tool, you will notice numbers at 2 of the corners of the select box, and if you're very good with math (i.e. know if the numbers showing are multiples of your desired aspect ratio) and have a steady hand, you can accomplish what you are asking for on the fly. Otherwise, you'll have to figure out those dimensions beforehand or one at a time, and manually try to match up the numbers. In the end and in all circumstances, in my experience, mathematics is simply inescapable.
photo crop PRO
You may want to demo using an application dedicated to exactly what you want to do (rather than seeking functionality that just isn't there from such a simple, although often very useful, application such as Preview.app), for example,
photo crop PRO.
I personally would not recommend using Mac App Store ;-), but it is
also available from the Mac App Store.
Alternatively, for an application far more powerful than either Preview or photo crop Pro, I've discovered darktable, which
"is an open source photography workflow application and RAW developer. A virtual lighttable and darkroom for photographers. It manages your digital negatives in a database, lets you view them through a zoomable lighttable and enables you to develop raw images and enhance them."
darktable can be installed manually, but is also maintained as a port using MacPorts, if you prefer to use package management. I've included step by step instructions for installing MacPorts and darktable below.
Another extremely powerful alternative suggested in another response (I thank you Ecnerwal!) is using the GIMP. The GIMP can be installed manually, or built from source using MacPorts. I've included instructions below on how to install the GIMP using MacPorts. If you know Adobe Photoshop well, you may want to demo using GIMPshop, which very cleverly and conveniently reconstructs the look and feel of the GIMP to appear and behave nearly just like Photoshop... and its all for free.
If you're looking for a mature package management solution, for OS X I recommend MacPorts. MacPorts requires an appropriate version of xcode; xcode_5.1.1.dmg is the most recent version for Mavericks and Mountain Lion, and will require registering for free as an Apple Developer. Once registered and logged in, the link above for xcode will initiate the xcode download.
Get to know MacPorts
Enter these commands using /Applications/Utilities/Terminal.app
curl -Ok https://distfiles.macports.org/MacPorts/MacPorts-2.2.1.tar.bz2
tar xf MacPorts-2.2.1.tar.bz2
sudo make install # *not war!*
rm -rf Macports-*
sudo /opt/local/bin/port -v selfupdate
add MacPorts to your $PATH:
sudo port -vsc install darktable
install the GIMP
sudo port -vsc install gimp
search MacPorts for other free photo utilities
port search photo
to completely uninstall MacPorts
If, for whatever reason, you need to remove MacPorts:
sudo port -dfp uninstall --follow-dependencies installed
sudo port -dfp uninstall all
sudo rm -rf /opt/local
sudo rm -rf /Library/Tcl/macports*