You select the number of entries in the table based on how "pure" the output needs to be and how fast you are able to index the table. The more entries the "better" the sine output will be but it require you to index the table at a faster rate to get the desired output frequency. If your maximum output frequency is 50Hz and you have a SIN table with 64 entries for a full a period you need to index the table and update the PWM dutycycles 50*64=3200 times per second. When the output frequency is 5Hz you index the table and update the PWM dutycycles 5*64=320 times per second.

/Henrik.