If it's the size of a pea, it isn't likely to be harmful. Technically speaking, a boil involves more than one hair follicle: on your nose, something the size of a pea couldn't really involve more than one follicle. So strictly speaking, the thing on your nose probably isn't really a boil at all: it's a pustule or maybe a nodule. But not a boil.
Most of these things go away on their own without any problems (except, of course, you feel mortally embarrassed by it!). Give it four or five days and see if it is getting smaller. If not, or if at any time it is getting rapidly bigger, go see your GP.
For anybody who does have a boil... they often need to be cut open by a doctor, or treated with antibiotics, to make them go away. Signs that a boil is causing serious problems include: sweating, fever, chills, loss of appetite, vomiting, severe tiredness, headaches, and feeling generally cr*p. If any of these things happen when you have a big boil, definitely see your doctor.