Without going off to the “tablets overheating” topic (use the link posted above for that) it seems fair to say to the OP that “any which will fit but make sure it can be cooled”. But also an Ipad is not needed; you can use a lower power tablet (which probably means not Apple).
I use an iPad Air exclusively with SD. This means I have no other apps running in the background.
Also, I don’t have the iPad fixed to any surface – it sits on my lap. This avoided overheating. Now, I’ve got AC on the plane, and that helps.
I never got into trouble. Having said that, I have an old iPad Mini as a backup, and I also have a paper chart. Can’t you tell I’m married to a lawyer? :)
iPad mini (think it is version 5 or 6 now… I usually use them for 2 years, then replace. Remember the price of paper charts?) is the perfect cockpit tool 
Using it in portrait mode displays VACs as well as IFR plates the “correct” way. The device is small and works perfect. SD works good too, and I can only recommend using the new “extend vertical profile” to visualize complex airspace structures.
Overheating can be for the most part be prevented by:
My iPad mini is my main IT tool, used for all things related to aviation, as well as domestic stuff, such as reading books, etc, etc. Accessory galore, though one must is a keyboard case.
I upgraded mine to a Mini5, because I use it for everything, and the 32GB memory was no longer enough.
I had a Mini 3 before. If it was dedicated to flying, it would have been plenty for SD. They are very cheap second hand.
Overheating is prevented by sitting above an airvent (RHS floor vent in a PA28, pointing straight up towards the iPad). Never had any issues even flying straight south midday in the summer. On the ground I take it out and into the shade after landing.
When I started flying in 2015 I had what was then a standard size iPad, and found it too big. Personal preference I guess.
I use an ipad air for ifr flights, on a yoke mount, almost never gets un thé sun in a pa28. For vfr short flights, i have all in the iphone, ok as soin as I don’t have to read a full plate.
Ipad air is too big for a pa28, but finally it’s not so bad and doesn’t overheat
Using latest version of the iPad mini (is it 7th Gen?) with SD. No cooling in location, more importantly NO direct sunlight exposure (close to EVER), using build in SIM, charging is almost always connected – zero overheating detected. I think the direct sunlight exposures plays the largest factor in overheating.
Wow… 110KIAS, good going for a Rallye.
I still prefer the iPad directly on my lap. With it mounted on the panel, on the center stack, it would be a tad foo far away / too angled for my taste, especially if I need to look up something in an AIP or so.
I was at first reluctant to go for the Mini, but in the small cockpit, I regard it now as a given.
Apple iPad Mini 6, mounted on the yoke (with a RAM Mount) works like a charm to me. However, I am running Garmin Pilot, don’t know the difference in power consumtion and processor needs. Important to be able to charge, make sure there is plenty of power in your outlet. Also, a power bank as backup is not a bad idea.
My friends use the Mini 5 and the processor speed is significant. I would definitely go for the 6 (they are about to upgrade).
Not SD, but hope it helps! :-)
Yeager wrote:
I think the direct sunlight exposures plays the largest factor in overheating.
My observation also.
I still prefer the iPad directly on my lap. With it mounted on the panel, on the center stack, it would be a tad foo far away / too angled for my taste, especially if I need to look up something in an AIP or so.
Me too, and it enables one to always move it away from direct sunlight which indeed seems to be the only reason for overheating. iPad mini 4, no sim, usually connected to power, running SD. But I have to admit that I have the luxury of a large map display of a Garmin G3X in front of me..
