Large Red Blood Corpuscles
Red blood cells (RBCs) are born in the bone marrow. When they mature, they are sent out to work in the bloodstream. An odd thing about cells in the human body is that they are biggest at birth and get smaller as they mature. 
RBC’s start out with a mean corpuscular volume (MCV)  of over a hundred at birth then shrink down to ninety. Two important requirements for maturation are Vitamin B12 and Folic acid. Both are necessary for normal maturation.
RBC’s are easy to see under a microscope. It is easy to see if they are immature (too large). But what about the brain cells and the immune cells?  B12 and folic acid deficiency also affects them but there is no easy test to diagnose their problem.
In summary, large RBC’s are a common symptom of vitamin B12 deficiency.
Worst Case Scenario
Over a hundred years ago, some people would become so B12 deficient that they would wither away and die of a disease called Pernicious Anemia. In addition to the anemia, patients would also have nerve degeneration (neuropathy) Nowadays however, they are diagnosed and given B12 shots which stops the disease. In the 1920’s, the only treatment of pernicious anemia was eating a half pound of raw liver daily. B12 shots are not as tasty but work better.
Warning. It was discovered that folic acid could treat the anemia but would not stop the nerve degeneration. If you have large RBC’s you must have both B12 shots and oral folic acid.
The therapeutic doses are:

• Vitamin B12           1000 micrograms intramuscular every one to two weeks
• Folic acid               one to five milligrams daily by mouth.