Was hospitalized for influenza b with low white blood cells, neutrophils, and platelets. counts have slowly been rising after starting tamiflu (oseltamivir). ultrasound revealed slightly enlarged spleen 13cm?

Nothing to worry: That is a usual observation with a viral infection and usually nothing to worry. But better check with your doc to make sure nothing else is going on.

Spleen only modestly: Enlarged and not uncommon with a viral infection. Low ish counts also seen with virus.

Improving?: It sounds like things are improving. I would not be too worried about an enlarged spleen at this point as this can happen with a systemic infection. However, it needs to be at least followed, like the blood work. Right now, the most important thing is how you are feeling clinically.