The 7970 is not overly large and should fit in a mid tower case , when you are looking at which card to buy they do give the dimensions of the card in the details tab.
As you can see in the details of this card it's 10.8" x 4.4" x 1.5" and for the most part the cards are 11" long so you will have to see if the hard drive bays are situated so as to put the hdd in a slot that is not directly in line with the video card. If the hdd bays are facing out and not towards the rear of the case then that is the best way , but if you only have one hdd and there are several bays then you should not have any problems.