Yes.... the larger is your explant, worst is the media (and gas) perfusion/exchange. So, as you increase the size of your explant, higher is the risk of cell death by hipoperfusion, specially those cells located in the middle of the tissue (central area). 

Thanks Litia, for your answer. I discover that majority of authors use hipoccampal slices. I was thinking maybe it has to do with the sensitivity of the hipoccampal cells over those of other brain regions and wondering whether there is a good prospect of publishing work on cortical slices.

its good to use hippocampal area as it is more vulnerable to ischemia/ reperfusion attack as compared to other areas.

alongwith that i agree with Litia