All of the different names for upright and grand pianos are marketing. A vertical piano around over 42" high is marketed as a "studio", "upright grand", "concert", "artist", "parlor" (blah blah blah.) -- then there is the spinet, which should never be considered on any level. In grands there are, "petite" or "salon" grands, "parlor" grands, "baby" grands, studio grands, concert, "Imperial", and on and on.
In the grands "petite", "salon", "parlor" was to give the impression that you COULD fit it in you home. In verticals, the "studio", "grand", "concert" desginations are to make you believe that they are something that they are not.
NO vertical piano is a "grand". The first distinction is the action. A grand has a relatively simple action which utilizes gravity. A vertical piano requires more mechanics and utlimately feel different and have difficulty in certain areas (particularly repetition). Vertical pianos also eminate their sound differently. A grand has space below and above the soundboard (the upper part being easily opened and closed.) A vertical is designed to fit against a wall. There is more wood (and not able to be opened as easily) between the front of the sound board and the performer. This can actually cause problems in situations (like a choir accompanist) where the piano is not against a wall. The difference in volume from the performer's side and the back side is substantial.
The ideal is a grand -- a horizontal piano with a modern grand action. That is not to say that a quality vertical piano, which cost half the price and take a fraction of the front to back space, can be a better option over a crappy grand.
For some basic comparisons, a "petite grand" and the like have a sound board and bass string length similar to a 37" spinet! Rarely (though there have been instances) are these worth the money -- they are decor, if you are serious and space is really an issue, get a good vertical. A 42" console is roughly equivalent in sound board and bass string length to a 5'1" "baby grand". Tonally, a 52" is heading toward the 5'7" - 6' grands -- they have sound boards and bass strings similar to these instruments.
In the end it is about the quality of the instrument (which name has a substantial bearing, but do your home work -- some early 20th c. pianos, like Knabe or Chickering, are different from mid to late 20th c. "stencil" versions), and tone quality. Yes, size matters, but from 5'7" on, the space is an important issue. A too big piano can be just as unsatisfying as a not good instrument -- a 9' grand in a space that would require no more than a 6' can in itself be problematic.
I will quit rambling now.