The first thing a scenic designer (who for the sake of simplicity we will call "Bob") should consider is the mood and spirit of the play. It is important that the design be expressive of these. By mood I mean the feeling of the play. Is the play comic, tragic, happy, or what? By spirit I mean the manner and the style in which a play is presented to the audience.
The next thing Bob must consider is the historical period of the play. A basic understanding of the general motifs and idiosyncracies of the period is necessary. This allows Bob to use them in a way that creates a faithful visual representation.
Next Bob needs to consider the locale of the play, the socioeconomic level and personality of the characters, and the season(s) that the play takes place (this is left mainly to the lighting and costume designers).
Then Bob must look at the elements of composition. First is always the line. Line defines form. The suggestive characteristics of the line defining any form will dictate the emotional qualities of that form. The character of the chosen line will make a perceptual key that helps explain the psychological nature of the thing being depicted. The next element to consider is the mass. The esaggeration of either form or mass can be used to stylize a design. Bob must also consider the elements of composition of the value and the color.
Lastly Bob must consider the praticality of the setting. He must remember the needs of the director by making sure his set design meets the the director's specifications. The needs of the actors is also important. As are construction demands of the design. Last but not least Bob must consider the ever present time and fiscal bugets. Bob is ultimatly responsible for the timely construction of the scenery and properties. It is the designers responsibility to produce the best possible design that budgets will allow.