Mine is about 6" and is in a 30gal (4ft long) tank. I've had it about 5 months and it has grown but slowly and is still very slender. It feeds well and is far more active than I thought it would be. I did have a pair but one jumped so it has hooked up with a young half banded eel (about 5" and definitely broader/rounder) they get along famously.
if you keep it in a small tank, then what'd you expect is a small eel. more room allows better growth. of course it depends on what you feed it with as well. from experience, spiny eels DO NOT take processed food. if yours do then you're really lucky. bloodworms, tubifex, brine shrimp, daphina is very much the staple.
Mine is fed bloodworm, daphnia and pretty much any other frozen food I can find (cockles, mussels, shrimp) it will also eat dried bloodworm and tubiflex. It is quite adventurous, it tried a bit of flake but spat it out in disgust!
if the shrimp's small, then yes. i wouldnt risk feeding amano shrimps to them actually, well at least not live ones. amano shrimps are very hurt able to hurt the delicate skin of the eel