How to give a typical studio dimension: split the levels, decorate on the diagonal

It could be a rather stylish studio apartment. Or it could be an upstairs retreat for casual tete-a-tete dining, televiewing, reading, conversation. ''Our idea was to take a rather typical smallish room with fireplace and make it as interesting as possible,'' explains Susan Garfinkel of East Hills, N.Y., a member of the International Society of Interior Designers. She and her partner, Carolyn Miller, decorated this room in that organization's design showhouse in Manhattan, and they term its style ''studied informality.''

They oriented the entire room toward the fireplace wall, with its splendid painting over the mantel and the corner television set. Diners face that wall, sitting in elegant French wing chairs pulled up to a contemporary dining ledge. So do the persons who sit in the modern upholstered seating modulars arranged snugly into the wedge-shaped floor area in front of the platform.

''We did the room on the diagonal in order to get a little more dimension. And the platform (which covers about one-third of the room) helped give more dimension, too. We feel it is better to make two levels of floor height in a room like this, rather than keep it all one surface,'' explains Mrs. Garfinkel.

It is a white-on-white room. The walls and floor are white, and the seating is covered with white fabric.

''It is about the sleekest mixture of design elements we could assemble,'' she explains. ''We loved mixing many different styles. We put a country pine table behind one segment of seating. We hung a glittery and ornate Venetian mirror and chose a neoclassic column as a base for an oversized vase. We covered the windows with matchstick roll-up blinds and then draped them with soft and inexpensive matte jersey, which is actually a fashion fabric which can be bought in piece goods departments.''

They defined the seating area with an area rug and chose a low coffee table that is lacquered to look like marble and is more practical than marble for heavy-duty use. The room looks ordered, uncluttered, and peaceful - and also much larger than it actually is.