Should I graduate to floor standers or will larger bookshelves suffice?

There is a middle ground between floor standers and bookshelf speakers.

Stand mounts they are called.

I own a pair of Reference3A DeCapo I "stand mounters".  Way too big for bookshelves, but half the height of floorstanders.  Surprisingly copious cabinet volume which of course helps the bass.  I think they are 10 or 12" midwoofers.

I tried Proac Floorstanders, they were magnificent but required too much power (this was before Class D and I am a Class A/Tube guy).

The DeCapos are 90/dbw and incredibly musical...

Bookshelves that are well made will thump just as well as many floorstanders. Genelec 8351B or Dynaudio Confidence/Contour lines, or ProAc. And if you really need the low end, which in modern city apartments is seldom missed on speakers like these, you can add subwoofers which add value to floorstanding too. Note that Genelecs are 3-way, as are Dutch & Dutch or Kii Three. 

If you have the space of a gigantic house to house floorstanding , sure. But the notion that you need that format for a "full range" sound is tediously passe. 

Good advice from @soix

Might check in with The Music Room, Ascend Audio or Zu Audio ... especially Ascend.  All have models in budget and have 30 day return policies.

Stand mounts are a good suggestion. Wharfedale, Mission  and others are offering throwback big baffle speakers that are selling at a wild pace…at Mid Fi prices. I love my Wharfedale 85th Anniversary Lintons, stand mounted with good cables and a speaker level output REL T-Series sub.

A very high book shelf will image better ,if you have a very high quality subwoofer then no problem for it’s necessary if not then a bigger floor stander Quality does count !