It's not uncommon for beds to be slightly shorter than the person using them. Furniture is not standardized at this time, so obviously most people could have had their beds made longer if need be. Another good example of this would be George Washington's campaign bed. We know it was custom made for him through the upholstery firm of Plunkett Fleeson in Philadelphia. At his death Washington was 6'2", his bed was only 6' (the bed survives at Mount Vernon). If he needed the extra length there's no reason he couldn't have had it built that way (the space in the tent allows for it as well). We also know he purchased pillows and bolsters from the same. Since it's unlikely he would have had the bolster for just decoration while on campaign, one can assume they had the use of helping to prop him up while in bed.

Looking at images from the 18th century of people in bed there's a variety of positions. Note that there are MANY images out there since so many interesting and comical situations occur in the bed, but I chose these because they all seemed at rest. Matrimonial Comforts, German Luxury, The Good Man at the Hour of Death, and The Wooden Leg. German Luxury shows well how a bolster pillow might be used, while The Wooden Leg shows one man propped up and the other laying flat. It's very likely it was a matter of choice, but there's certainly plenty of visual evidence out there showing people sleeping propped up. Medically, it does help with breathing and the lungs, something more necessary in an era of constant fire burning, dust, and other air issues. It also doesn't take much of an angle to reduce the persons full length, so it's not like they would need to be straight upright.

This is a difficult question, because the answer rests entirely on negative evidence. While some individuals may have felt that sleeping while sitting up in bed was preferable, there are no medical texts that advise all people to do it. Rather, people with respiratory issues were supposed to sleep with their head raised (as today). In texts on 18th century furniture, academic authors do not mention anything about beds being significantly shorter.

But the fact is that the claim that people slept sitting up is the one that requires proof. It's kept in currency by millions of docents, but it's based on nothing but visually estimating the length of antique beds, which can have the illusion of being smaller than they are due to the size of a canopy and hangings or the pillows put on it. Without the standardization of mattress sizes, beds could be made to any dimensions, which means that, in theory, some beds may be shorter due to a reflection of the height of the owner.

If Massie didn't provide a citation or explain that he'd measured beds to find that they were significantly shorter than a human body, then there's no reason for the previous answerers to need to take it into account. It's not uncommon for academic writers to not apply the same rigor to statements they make that are outside of their subject matter, especially if said statements reflect attitudes taken by most as a given.