I am trying to make a CAKE SIZE "Blueberry Cake Donut"... Can I fry that?

Here's another recipe...a jelly donut (sofganiot) cake with raspberry filling.
Switch in blueberry filling and maybe it will be good for your friend's birthday.
https://food52.com/blog/11887-a-giant-jelly-donut-cake-for-hanukkah

I know it may not specifically be what *you are after, but the idea of a blueberry cake donut holes croquembuche will not leave my mind now.
As to your plan, I'd look for a recipe of a roll-and-cut-out style dough. (See Erin's article https://food52.com/blog/17706-cake-doughnuts-101-the-completist-s-guide-to-shaping-glazing-frying-baking-your-own)Then, you can make it large, but not too thick, so it should still cook quickly. Or do like Manisha suggested, and start normal sized and see how big you can get, the you can make an impressive layer cake with all the tiers.
Whatever you pick, be careful around the pot of oil!

Maybe a blueberry cake doughnut recipe in two bundt pans. Invert one and put the other on top...boom! Giant doughnut.

I would use donut batter, bake it and then drop it in oil after to crisp up the outside. Might work, might not! But that's what I would try.

Maybe you could fry "funnel cakes" of the batter, then stack to make a cake?

How about some blueberry donuts as a present and then buy or make a more traditional cake.

another thought, some of the issues I see with this: Possibly safety issues, (see Merrill's post about heating up a huge pot of oil and leaving it on the stove) here https://food52.com/blog/20896-how-we-holiday-we-cook-1. Also, putting such a large object in oil, the outside may overcook before the inside gets done. Plus you have to keep the boiling oil at a consistent temperature. It's nice of you to want to please your friend, but will he notice the difference between an actual cake and a doughnut? and, if he specified that it should be fried, unless he's an avid cook, he might not be aware of the challenges this entails.

I agree with Berry Baby. There are some things which are just a better idea to purchase.

I think u should start with regular size donut then increasing its size. If regular size or other increased size will fry perfectly then cake size donut will also fry.
Practice makes a man perfect :-) good luck.
Whenever you will succeed just upload its image on this post.

I think you will have a lot of difficulty with execution if you try to fry it. I'm assuming you're talking about this recipe by Erin McDowell: https://food52.com/recipes/28779-blueberry-cake-donut-bars. Since it's designed as a cake batter, it's going to be best baked as cake. I don't see how you will get a liquid cake batter to hold together as a doughnut, and especially as a single large doughnut. The idea of carrying out such a large sized fried object creates its own set of problems even if it could be done. Why not just bake Erin's carefully crafted recipe as she developed it?

I don't have an answer, but I have all kinds of respect for this question! Good luck!