upgrade tank

So, got this new (to me, used for 1 year by the people I got it from) 48 x 13 x 21. Has all the equipment needed (heater, filter, lights) although later on I probably will change the lights. But it's ready to go. First thing I need (besides cycling it of course) is a stand. Still chasing...

I have two BlackGhost Knife they are about 7cm in size at the moment. I know my 20gal Tank is not the right tank for them so i've been planning to get a bigger tank, but how big should i get? is 75gal enough for them? I know my house is small but i want to give them the proper tank. Please...