A big upright Piano like the Yamaha U3 is 131 cm (51 1/2") high.
A small grand like the Yamaha C1 has a depth of 161cm (5'3") (and there are even smaller grands).
The depth measurement for the grands includes the keyboards, so I guess the C1 and U3 have a pretty similar size on their soundboards?.
So, from what size does a grand piano make sense compared to a nice upgright piano like Yamaha U3 or U1? Something like the C1? Or bigger, perhaps like the C3 186cm (6'1")?
It might seem odd to the purely objective observer but, in my experience, at least, most piano buyers do not make their buying decision between grands and verticals purely on the basis of tone performance. If this were the case large vertical pianos would outsell small grand pianos but, at least in the product line I’m most familiar with, they do not.
People buy small grand pianos for a variety of reasons; aesthetics being high among them. Action performance is also high on the list.
Many years back in the bad old days it was true that most small grand pianos—those shorter than, say, 160 cm (5’ 3”)—sounded pretty bad. Their scales were unbalanced and, in a misguided attempt to obtain more “power” excessively massive hammers were used. Not only was the lowest half-octave pretty much useless, the whole bass sections often sounded dull and tubby. The bass-to-tenor transitions were usually rough and uneven sounding. These were, by most every standard, un-musical atrocities that had rightfully earned their somewhat derisive Piano Shaped Object
But, that was then and this is now.
Times have changed. Piano designers (and manufacturers) have taken another look at these instruments and the modern small grand piano is often quite nice. Despite what the traditionalist might tell us, there is a growing selection of relatively short—say 175 cm (5’ 9”) and below—grand pianos now available. You may have to look beyond the large, traditional manufacturers to find them, however. And some of them—to cite just one example, the Walter 175—will hold their own against traditional pianos that are significantly longer and larger (and, for that, more expensive).
As well, there are a growing number of 150 cm (4’ 11”) grand pianos that are earning reputations as credible musical instruments in spite of their diminutive length. And in spite of their often very low prices. There is a growing selection of very nice short grand pianos available for less than $10k. It might pay to broaden your search.
The market has changed and it is no longer accurate to simply write these pianos off solely because of their length.