As expected, this is a horribly hard MU.
In your favor, Zard has an amazing jab and one of the very few f-airs with more range than Sheik's. Decent, constant high damage output through dthrow->fair is a decent asset against any Sheik. You're heavy and hard to gimp. A single read from Charizard can take very early stocks from Sheik (upB airdodge read being the most prominent example).
On the other hand, there is an abyss between both characters' frame datas. You can't touch a good Sheik, she punishes your every attempt to go on offense and Charizard's lack of aerial mobility is a humongous throw-off against needles and good spacing.
Play ultra safe. PS->grab her aerials if they end up being mispaced. SH Nairs are slightly less safe in this MU than in others but they still work. Be careful with using downB to survive when dthrown at high percents, as her multi-hit uair and invincible, delayed upB can both hit non-super armored frames.
Dedicated Charizard mains could test whether up-angled ftilts beat Sheik's SH aerials. Sliding Rock Smash can surprise a SH-happy Sheik. Good thing your grab outranges hers, though grab loses to anything else in her arsenal. If Sheik is forced to use her recovery to its limit distance-wise, it's not hard to learn the timing to dair her vulnerability frames. One may also test completely ridiculous ledgegrab punishes like RAR rock smash, anything's a plus if it can work against the character with the most options at any given time in the game.
Stages to pick? She has deadly setups on Battlefield and Green Land, and can run from you easily on FD/Omega. While benefiting her uair and upsmash, stages with low ceilings (and, to a lesser extent, Duck Hunt and its top tree platform) can work your way through uthrow cheesing.
I'd say this MU is a heavy -3. Most of a Charizard's best bets against Sheiks may be their unawareness of the match-up and rage effect shenanigans.
