I understand that low-grade lymphomas don't progress as fast as higher grade ones, but do the lymph nodes still enlarge as fast. My friend was told that they may have to treat him sooner if the nodes get too "bulky." With his being low grade, wouldn't it take a long time for this to happen or does it not correlate with the grade?

Pathologists tend to classify lymphomas as either low-grade or high-grade. Low-grade lymphoma tend to grow much more slowly than high-grade lymphomas. Low-grade lymphomas tend to grow over the course of months to years; high-grade lymphomas tend to grow over the course of weeks to months. Low grade lymphomas can indeed get bulky since they can grow over a long period of time without a patient knowing it since low grade lymphomas often don't make patients feel sick. A high grade lymphoma tends to make patients feel ill as they grow (fever, chills, night sweats). These are general rules-of-thumbs; there are exceptions of course. Often lymphomas can be low grade and transform over time to high grade lymphomas.