Measure both the width and the depth of the floor space where you'd like to place a desk. The easiest way is to use a measuring tape. Hold the start of the tape down at the edge of the area, and let it run to the other edge. Make sure the tape is parallel to the side of the area, as otherwise you would overestimate the available space. If the distance is larger than what you can stretch to with your arms, ask someone else to help, or split what you need to measure up into multiple parts. You can do this by either holding the tape down halfway and continuing running it while letting the first end go, or by marking how far you measured on the floor or wall, and reposition the tape to complete the measurement. If you want to avoid drawing on the floor, you can position a pointy object like a screwdriver there instead. Don't forget to avoid dead places where the piece of furniture won't go like skirting boards. Also it may be necessary to take the height of the space and the item into account wherever the ceiling is uneven like in loft rooms.