Would you stop methotrexate or leflunomide if MCV increases after starting the medication with no other parameters of anemia or leukopenia?  

I would take this as a sign that the patient is taking their medication. Pretty common and definitely wouldn’t withhold RA tx for this.

I would not recommend stopping the medication based on one single parameter in the blood count. I would repeat it, check for anemia or other cytopenia, and depending on if the MCV increased or decreased, potentially send a further workup for iron ferritin, vitamin B12, and folic acid.