Eggs just to summarize, there have been a lot of great answers and suggestions so far.
A 1.50” tube will be better since you’re going for length. I have about the same girth measurements as you, and do not pack the 1.50” tube. For the 1.75” tube, we would need to gain almost an entire inch of girth before we could even hope to start packing it.
You can still gain length even though you are not longer in the tube. Certainly it is not a great sign, obviously we would be better off with instant extra length. But when you come out of the tube and do your jelqs, I suspect that your unit is actually longer in this state than it is normally. So you are jelqing a longer penis. That is how it is for me, and I am also about 1/8” shorter in the tube. Consider the balloon analogy for the tunica. The balloon is not expanding lengthwise in the pump, but it is getting stretched girth-wise. When you take it out of the tube, the balloon has just been stretched out, and with that extra elasticity it stretches farther in the length direction as well as girth direction during the jelq.
As the months go by, you are able to stay in the tube for longer periods of time / more sets / higher pressure. And those translate into more length in the tube. Especially time in the tube, as several have mentioned.
Finally maybe it is just not your thing. Some people gain length from pumping, other people don’t. Just from the genetics and anatomy of the individual. But don’t write it off yet, put all those ideas to work for a few months and see if there’s any progress.