When buying a whole house water filter size is important.
One aspect of water treatment seldom mentioned to consumers is contact time. Incoming water must remain in contact with the carbon in your filter for a certain amount of time in order for contaminants to be removed. In the case of chloramine, which is very difficult to remove from water, a 2 cu ft whole house water can completely remove chloramine at a flow rate of 3 gallons per minute (gpm). If the water is flowing faster than 3 gpm some chloramine will remain in the water. So the size of your whole house water filter is important to removing contaminants.
Another issue is that cities will add more chloramine in the summer months and less in the winter. The amount of chloramine in your water also matters.
Most whole house filters are tested for the removal of chlorine, which happens to be the easiest contaminant in water to remove. NSF42, an aesthetic certification touted by water treatment dealers relates to the reduction of chlorine.