Gas and Oil Home Heating Furnaces - what size unit?

I am replacing a central ac/gas heat package unit. The a/c is 3 1/2 tons and I don't know the furnace size as it is rubbed off and it's been 7 or 8 years ago since we installed. My question is my contractor says to down size to 3 ton a/c, didn't say about the furnace, although I believe the furnace is too big for the house, 1200 square feet. When the furnace would come on it would run just a few minutes then shut off and come right back on in just a few minutes, it repeated this all the time except if it was 20 degrees or below, which doesn't happen a lot in south Mississippi. The a/c comes on and stays on and never goes off till late at night. It stays fairly cool in the house except for the front room which has 3 large windows facing full sun all day. Seems I don't need to downsize the a/c, does anyone have an idea?

Do yourself a favour and have a heat load calculation done on your home. That is the info you'll need to determine heating and cooling. Your contractor should be able to do one, or your utility company.

is correct, have a heat load done on the structure. to determine BTU of heat multiply # of burners by 25KBTU, if trane, multiply by 20KBTU. this is a rule of thumb only