They are rarely satisfactory. A good way to test your material is to stand 1 foot away for every inch of original copy width. If you can't read it from that distance, then your audience will not be able to read it either when it is projected. If there are any more columns or rows, the information will not be readable. Whenever possible, present data using bar charts or graphs instead of tables.
Colored graphs are very effective.
Do not show data points unless scatter is important. Color adds attractiveness, interest, and clarity to slide and viewgraph illustrations and should be used whenever possible. If you use color, remember that contrasting colors are easier to see. Be sure that they are clean and in good physical condition. Critically examine every slide and viewgraph and try out the entire set under adverse light conditions before using them at a meeting.