Using paper, pencil and a ruler plan out the rough design of your table.
Use brainstorm sketches first, don't worry about dimensions.
Once you have settled on a rough design, pencil in some rough dimensions. Your dimensions will vary based on the kind of table you intend to construct. In other words, a dining table will have different dimensions than a bedside table.
Consider where you intend to place the table when creating your dimensions. It doesn't have to be any size other than the size you need.