A good quality grand piano has substantial advantages over any upright due to its design. There are two aspects to this, firstly the tone design in a grand piano means that the sound naturally comes out from the piano both downwards towards the floor and from the lid, even if it is only partially open.
In an upright piano the sound is enclosed in what is essentially a box, and as they are normally placed against a wall, the sound coming out from the soundboard is restricted immediately by the wall.
Secondly, the mechanical design in a grand piano allows for much greater control and for the pianist to develop their technique to a higher standard. This is mainly because the hammer comes back from the string naturally with the force of gravity which allows for a mechanism that offers much better speed and reliability of repetition and allows for smaller increments of touch control.
The grand piano mechanism makes it feel as though the sound of the piano is almost directly connected to the tip of the finger and this changes the way people play. For anybody who is undertaking exams or performances, whether professionally or not, there can be a substantial jump from practicing on an upright piano to doing an exam or a performance on a grand. This may affect people’s performance in such situations far more than they realise.
One could go so far as to say that the piano IS the grand piano and an upright piano is a substitute dictated by space or budget.
Size is often misunderstood, with people assuming that it is directly related to power and the space the piano will go into. In fact, it is mostly related to the quality of the tone, not volume. Longer strings and a bigger soundboard give a piano a richer, more complex and more pure sound. Small pianos, depending on design, may sound indistinct and have a limited tone due to the short strings.
This is particularly important in the bass register. Bass strings are made with a steel core (as found in the middle and treble strings) with one or more layers of copper wire wound onto it. The copper adds weight, which is needed to produce a given pitch with a particular length of string. As a string gets shorter additional copper must be added to sound the correct pitch. This weighting unfortunately makes the tone less clear, with harmonics that are not as pure as found in a longer, lighter string.
Available physical space may well be a factor, but piano sizes go up in small increments, often only a few centimeters. These few centimeters have little effect on the space occupied, but considerable impact on improving overall tone. So when looking at piano purchase options, ideally you want to choose the largest size that space and budget will allow, assuming all other factors are taken into account, such as age, brand quality and design.