Check with the manufacturer of your baseboard. They will be able to tell you how many Btu's per foot it will put out at a certain temperature of water. For example, typically you will get 580 Btu's per foot at 180 degree boiler water with Sterling standard baseboard. Calculate your room area in square feet. Typically you will need 20 Btu's per square foot so a 12x20 area (240 sq.ft.) needs 4,800 Btus. Or 8.25 feet of baseboard. Areas of the world do figure it differently so check with a local heating contractor in your area for exact calculations.
You can always bump up your boiler water temperature to see if it can accomidate your heating needs or you may have a valve that is restricting the water going into the baseboard.