Grand pianos are measured with the lid closed from the very front of the piano (keyboard end) to the very back (the tail). Lengths start at 4' 6" and go to over 10' (even longer in some experimental models). Widths are usually around 5' and heights around 3', but only the length has a bearing on musical quality.
Grands less than 5' long are the musical equivalent of spinets and consoles; that is, they are musically compromised and are mainly sold as pieces of furniture. Grands between about 5' and 5½' are very popular. Although slightly compromised, they can reasonably serve both musical and furniture functions and are available in many furniture styles. (By the way, piano professionals prefer the term small grand to baby grand. Although there is no exact definition, a small grand is generally one less than about 5½' long.) Above 5½', pianos rapidly improve, becoming professional quality at about 6'. Pianos intended for the home or serious professional top out at about 7' or 7½'. These sizes may also satisfy the needs of smaller concert venues. Larger venues require concert grands, usually about 9' long.
三角钢琴的长度从4英尺6英寸起，到超过10英尺（甚至更长一些的实验型号）。宽度通常在5英尺和高度约为3英尺，但只有其长度对音乐品质有影响。三角钢琴长度小于5英尺相当于小型立式钢琴和游戏机的音乐，他们在音乐上有妥协，主要作为家具出售。三角钢琴在约5英尺和5英尺半之间很受欢迎。虽然音质轻微受损，但无论从音乐和家具功能可以提供合理的服务，有许多家具样式可供选择。（顺便说一句，钢琴专业人士更喜欢把小型三角钢琴称为婴儿三角钢琴，虽然目前还没有确切的定义，一般认为是一个小于5英尺半的三角钢琴）。5英尺半以上，钢琴迅速提高，在大约6英尺达到专业品质。用于家庭或严格的专业顶级的钢琴约7英尺到7英尺半。这些尺寸也可满足小型音乐会场地的需求。更大的场馆须音乐会三角钢琴，通常大约长9英尺。
（摘自：《PIANO BUYER》）