I understand you, especially if your hematologist is dismissive or cryptic.
I was having an ultrasound of my liver and on the spot I just asked the radiologist to check my spleen too. He was not too pink as he could not report on this finding to the doctor who sent me for this scan, but I got my answer and it was good for me to know.
I also got an abdominal MRI for another health issue and the radiologist mentions there are no abnormalities with the spleen in that report too.
I agree that it is good to know where you are in your illness, and spleen and liver size need to be confirmed by radiologist, not just by palpable enlargement of the organs. It can inform your treatment and, in some cases, risk and prognosis. And it can explain symptoms such as abdominal pain and feeling full after taking a couple of bites of food.
We all have different needs regarding management of our illness, and having accurate information is part of those needs.

I have had ET and now PV since 2001, and my spleen has been enlarged all of those years, at every checkup.

I’ve never had pain or any issues associated with it.

My hematologist tells me this is part of having these MPN’s.

I changed my doctor (hematologist/oncologist) earlier this year to someone more specialized with MPN. She recommended I get an updated ultrasound to check the size of my spleen (which I did earlier this month). I was getting more abdominal discomfort recently. She did palpate and indicated my spleen is large. I had previously gotten an ultrasound back in 2021 as part of the initial diagnostics of my condition, and it is much larger now.

My prior oncologist was not very concerned about spleen size if it was not really bothering me. However, my current doctor cautioned me that letting the spleen get too large is not good, especially if/when I need a stem cell transplant, and highly recommended Jakafi to start reducing it.

A better choice for determining the size of internal organs is a radiologist. You need an ultrasound or another type of scan (e.g., CT scan, MRI) to have an adequate measurement of the spleen. If your hematologist does that themselves, the ultrasound for checking the exact size of the spleen, then that is OK. Just palpating the spleen or the liver is not going to give their actual size. And sometimes it is better that you have an initial spleen size at diagnosis, even if it is within normal size. It can be used later to track eventual changes.

I have myelofibrosis. My Mom had it. So far my spleen is okay (diagnosed in 1982) My Mom's spleen had to be removed (this was back in the early '80's.). It weighed 8 pounds. She took an oral drug, the name of which I do not know), and actually got over her MPN. Her hematologist/oncologist was in partnership with two other specialists who together had 40+ years of experience. Her specialist had no explanation for her being disease free. Back when I was diagnosed, no doctor would acknowledge that MPN could be familial. This position has changed.