For your CNL project- the slide needs to be set to a specific size. 
Your posters should be sized at 42 inches wide by 36 inches tall. 

To make your slide this size in Powerpoint, you'll want to head over to the "Design" tab. 
Some Mac users will need to go to the "Themes" menu to find the slide setup options. 
Hint: If you're on a Mac and don't have a "Design" tab--- look for Themes.

From there, you're going to click on "Slide Size" or "Slide Setup" depending on your version- use the little "down arrow" to select "Custom Slide Size".  You'll choose "Custom" and enter the desired width and height.

The following animation loop will show you the correct steps to create a blank slide of the correct dimensions on a PC running Powerpoint 2016.  Other versions would be really similar; though I would personally advise against using a web-app version of Powerpoint- the one that runs in a browser.  Best, instead, to use a locally installed version of the program! 

Thom's email is tpinho@hshsl.umaryland.edu.  You can always feel free to email over a copy of your completed (meaning: you've got all your content in there) poster for a quick email consult.  (Emails before 4PM will almost always be answered before midnight.)

As always- feel free to reach out if you need a hand!  Email is, by far, the best way to get a hold of me (and remember to send me your files!) 
I am more than happy to take a look!

NEW for 2021

I've gone ahead and made a couple of YouTube tutorials to help out with this process.  

First up is what I'm calling Design with a big D.  This is mostly about the choices you make in creating a poster- what makes for effective visual communication and all that jazz.  I've been doing exactly this for more than two decades now- and while we're much more visually sophisticated than ever the fact that we know more and are conscious of decisions can make the vast array of options and choices available dizzying.  (I seriously love that clients have opinions on Futura versus Proxima Nova- honestly- I mean that without a shred of sarcasm)  Hopefully, the contents of this video help a bit in that area.

Design Principles tutorial

Next up, I've got an epic recording of me creating a poster from scratch in PowerPoint.  Basically doing exactly what you're tasked with doing.  I made a basic poster.  (Looking at it now- it doesn't push any boundaries, but it definitely is aesthetically pleasing and clear and concise.)  With apologies- it's almost two hours long.  I went in with the assumption that someone with even a casual relationship with PowerPoint should be successful with my instructions- so you might need to do some fast-forwarding or watch at double-time around some of the basic setup parts.

Epic From-Scratch tutorial

Oh, and because I mention it in the video, here's a transparent (.PNG) version of the SON logo fit for popping into PowerPoint- the very same one I used in the tutorial.  You can right-click and do a "save image as..." to grab it and use it yourself. 

(And as a bonus tidbit- the PMS in the file name, in case anyone notices and raises an eyebrow- it means, "Pantone Matching System"- and it's a reference to the color guide used for printers to make sure the yellow and red in our iconic logo is always consistently displayed.)