Size is Important... _ You should be able to pick up and set down a cup without excessive bending or stretching. The average occasional table stands around 16┰ inches high. Then you have the length to think about. A table the full length of a sofa would be too long _ about 50 _ 60% of the sofa length works well. Round, square or oval it's really up to your personal preference, but if you want to be controversial, you can do a lot with angles and curves, and there is no reason why they cannot be combined. So lose the idea that your table must be rectangular, round or oval.
Coffee Tables and End Tables: Placement _ The end table should be placed right up beside the arm of the chair. Not directly in contact, because any subsequent movement of then chair could easily shake the table and spill the contents of any container. Coffee tables should be placed 18" from the sofa, enough room to enable you to walk between the two items of furniture, but not so far as to place your cup out of each. Some have been known to use these tables as accent pieces, and place them away from the proximity of sofas and chairs. Quite frankly, there is little point in doing this because these tables have been designed for a specific purpose. If you want accent tables, there are many of them available to choose from online that look more of an accent piece than regular coffee and end tables.