Handbrake is an excellent free video encoder that is available for Mac and PC.
The main settings for Screenlight are highlighted below. You'll want to select the MP4 file format and web optimized to ensure that the moov atom for the video is placed at the beginning of the file. The video codec should be H.264 and the framerate should be same as source (unless you have a frame rate higher than 30fps).
In the example below, I've selected an average bitrate of 2500 kbps. You may want to select something lower if you are concerned about the speed of your reviewers Internet connection. Two-pass encoding will give you the highest quality output. Selecting turbo first pass can speed up the encoding time slightly.
You'll want to use the main H.264 profile and level 3.1 if your video is 720p or smaller. For higher resolution output, you'll want to use high profile and level 4.
For audio you should select AAC (CoreAudio), stereo, and a bitrate that suite your needs (generally 192kbps or less).
Once you've selected the main settings in this window, you should click on the picture settings icon at the top of the window. This will bring up an additional window where you can resize your video.
If you are going to keep the output the same size as your source, then I would select Anamorphic Strict in this window. This will keep the same size and aspect ratio as the original.
If you are resizing the video, you'll want to select Anamorphic Loose in this window. You can then use the up and down buttons to adjust the width and height. If the width and height of your target size is evenly divisible by 16, I would select Modulus 16.
If the width and height isn't even divisible by 16, as is the case with a height of 960 x 540 (540 / 16 = 33.75), I would select Modulus 2.
You can probably leave cropping set to automatic. Handbrake automatically detects black boxes that surround letterboxed video and crops them out. If for some reason Handbrake's automatic choice isn't what you want, you can customize it.