How large is a 1/700 scale IJN destroyer or cruiser?

1:700 scale means that 1 unit on the model represents 700 units on the real thing

A Kagero-class destroyer was 118.5 meters (391 feet/ 4692 inches) long.   At 1:700 scale a model of a Kagero is 6.6 inches long

The battleship Yamato was 862'11" (overall).   A 1:700 scale model of the Yamato is 14.8 inches long.

In terms of detail - it varies by manufacturer.   The Tamiya kits and most of the newer PitRoad kits have a lot of small details.   Hasegawa is also good. Older PitRoad is mostly fair.   Some off the old Fujimi kits range from fair to poor.  IMO Aoshima is poor to terrible.

There are aftermarket photoetch sets available for these kits available from Gold Medal Models,  Tom's Modelworks, and some of the Japanese companies such as Joe World and Leviathan.   

Remember that you are building to suit yourself and the only person you need to satisfy with the project is yourself.   If you don't want photoetch details -- don't add them.  That being said, PE does add certain small details which cannot be acconplished by the limits of injected plastic technology.

The detail always seems to vary.  The appearance of two kits from the same manufacturer can vary, too--which can be disappointing.

I want to remember that one of companies, and I think it was Skywave actually had some kits of nothing but IJN armaments, which meant you could at least get a uniform appearance if you had, say, several ships in aport setting, or a raft of vessels ganged up on one mooring.

Now, there is one to keep a hard eye on in 1/700--it's way too easy to put ships far too close together in that scale.  I know, I've done it before, and knew better, too. It's easy to forget that 500' (about 8" at 1:700) can be a "near miss" at sea (underway replenishment, one of those high "clench factor" manovering jobs asea takes place at 150 -200' seperation (and that's scary close).  At 1:1700, 175' is 3", which looks far apart for a couple of 6-7" long DDs.