Record DSLR Video at higher resolution?

Hi there geniuses - I'm a total newbie when it comes to this. I am trying to record higher quality video using Screenflow to capture it. At the moment I have a USB cable connecting my camera to the computer and it only gives me the option to record video in the format of 1280 x 720, and I need higher quality video capture. So I plan on purchasing an HDMI cable to connect my computer to my MacBook Pro. Will I also need to purchase a Magewell USB Capture HDMI Gen 2 in order to capture higher quality video from my dslr through using Screenflow to capture the video? Or could I capture higher quality video from my DSLR using just the HDMI cable. Also I am using Canon EOS Webcam utility to capture the video from the dslr. 
Thanks in advance! 

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