How do I calculate the size of electric baseboard heater needed?
There are all sorts of elaborate calculations that you can make to determine the total wattage that you need.  These take into account where you live, insulation, room volume, number of people, etc.

The quick and dirty rule of thumb many electricians use is 10 watts per square foot.  So if your basement is 1200 sq. ft., you will need 12,000 watts of baseboard heaters.  When wired for 240-volt operation, a baseboard heater puts out 250 watts per foot.  So a 6-foot long heater, for example, will give you 1500 watts.

Figure out the area (square footage) of each basement room, then multiply by 10 to determine the wattage you will need.  Next, figure out how you want to layout out the heaters.  They come in increments of 2-feet, from 2-feet long to 8-feet long.  If you have any windows, the heaters should be placed underneath them.

Last, decide how you want to control the heaters.  Baseboard electric heaters can have individual thermostats installed in the end of each unit or can be controlled by a wall thermostat.  However, if you go with an in-line wall thermostat, you will be limited to a maximum of about 3840 watts (about 16 amps at 240 volts).  This might work fine if you have two or three small rooms.