welcome to hcm.
of that size may be abscess
,blistering disorder of skin.
to comment about it few questions need to be answered.
like -is there any history of trauma/is it spreading/any discharge (eg.pus) is there/any such blister in other parts of body?
after all it need examination of that part for diagonosis.
but some general measures should be followed
1.keep the area clean
2.warm compress can be given
3.avoid trauma to that site
hope this will help.
if it increases/don't heal gradualy consult your family physician.