I have this identical question and honestly I am not sure we'll get a solid answer until someone gets a 14" in their hand and starts sticking things in the port to find out the depth.
I have an older 2013 MBP with a JetDrive that I desperately loved and relied on for external storage for simple files, photos and small backups.
Though a couple 14" MBP review videos have come out for the 2021 model and they have some images?
https://www.youtube.com/watch?v=23ys9v1CGLk @ 2:22 mark.
Tom's Guide put in a SanDisk 128GB SD Card in his 14" and you can see it where the port cuts off the SD card; right above the 128GB words.
Thank god SanDisk has a pretty universal sticker design across most of their products, so you can probably try to guess the depth based on dimensions on the cards Amazon page. 0.09 x 0.94 x 1.26 inches
Some quick napkin math and using any image editor to compare pixel counts of the product image. The depth is about ~0.63 inches. Which is probably closest to the JetDrive Lite 330, which is suppose to be 0.66 inches in depth. Which should fit about right. It was made for the original 13" MBP, I imagine the 14" is similar in size.