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?

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