What is a good size generator that will be just enough to keep my heat and refrigerator running in case of an outage?

Difficult to say without knowing the rated consumption for each of the appliances you want to power. Fridges and heaters generally take quite a bit of power to run, which means a fairly decent genset. Using one that only just does the job will mean that it will be working its guts out every time you use it, and that can lead to big problems. That being the case, it would be better to go for one that will power the entire house.
List all the appliances that you would normally use in the course of a day and add up their wattage. Some of your appliances may only be used intermittently so you should stagger their use. (Note that appliances with electric motors often draw more than their rated wattage at start-up.)
We have a small genset to provide power for lighting as we frequently have outages in our area, and during those we try not open the fridge unless we really need something - like a cold beer. This genset can also be used for power tools down the back of the block, or taken away on camping trips. If the power is going to be out for more than 24 hours it's time to crank up the big beast - 5·5kw - though we haven't had to do that yet....touch wood.

Yea what heater, central heat, or space heater ? Is it possible to heat with wood ? If not i suggest maybe making a one room wood heat room like in hurricane prone areas they will tell you to make one room an ac room for small generators.

As for a refrigerator most can be run with minimal gennys but i also again suggest a cooler and a freezer.
A freezer is easier on the power consumption once frozen.

But all my suggestions are for more than a weeks time as i do not think you will have that much more gas if you get into a long situation.

If solar is possible i would suggest getting involved in it. For about the same price as a good generator you can get about 300 to 400 watts of solar power which should be great for lighting and a small freezer.

The great thing about refrigerators or freezers is that you don't have to run the generator 24/7 to keep food chilled or frozen.

as far as heat never plan on using a generator for this purpose.Use a wood stove.You will go threw your fuel at an incredible rate.