This simple but very useful font is designed studs up. This makes it an ideal candidate for signing mosaics in a pretty non-intrusive manner. The fact that the font has the quality lending it its name - it's Compact! - is an added bonus.
Each letter is 5 studs high and 3-5 studs wide. Since the font is so simple each letter can be constructed by many different pieces, but I recommend using plates at least 1x2 whenever possible. This ensures that each plate is straight - 1x1 plates have a tendency to wobble, which results in jagged lines.
This is what Compact looks like written in its own font.