If your using a normal size tennis ball then I don't think the size would be a problem. I use mini tennis balls with Holly but normal size ones do fit in her mouth and I'm pretty sure she can run fine while carrying them.
Holly is a PugX, weighs 6.5kg and is 28.4cm tall at the shoulders, so a small dog. Your dog looks like he must be bigger than that (much bigger).
Also, I used the slit in the tennis ball to teach Holly to fetch and it worked fine. I just started handing the ball to her and then letting her drop it into my hand so she couldn't run off with it (she is a toy distroyer so I'm pretty sure she would have gotten the treat out herself given enough time). Then small throws inside, then small throws in garden....larger throws in garden (and also phased out the treat in the ball by giving her a treat from my hand).....small throws at park.... larger throws at park (but I still don't throw the ball far because I'm really not good at throwing balls
and Holly doesn't love fetch so there isn't much point in getting a ball thrower).
Keep your criteria consistent. Eg, if you want the ball returned to your hand then accept nothing less, if you accept the ball being droped on the ground next to your hand then the distance can easily grow and grow until your back to not having a retrieve.