You can right-click on an image file and select properties, then select the details tab. The image size will be shown in pixels.
Also, in windows explorer find your saved image and change the folder view to details. Right-click on the details header and check "Dimensions".
You can use Google image search - https://www.google.com/imghp?hl=en&tab=wi - to help find different sizes of images you have by dragging your picture into the search bar. You can then filter the search to just show HD files.
You might want read about the difference between PPI (Pixels Per Inch) and DPI (Dots Per Inch).
It's a little confusing, but basically printing something at 300 ink dots-per-inch doesn't mean that a 100 pixel-per-inch picture will be clear at all sizes. You'll be printing big clear square pixels. Anyway, the higher pixel count the better it will print. Find the highest pixel resolution picture you can and print a piece of it to the scale you want and see if that's acceptable to you.
Also, the picture you fell in love with is copyrighted to Check 6.
They will have a full resolution version for purchase most likely and may allow you to print it with permission.
This is another of theirs that is pretty amazing: