Upsizing battery won't have any negative effect. But there's no benefit either, other than it will have more battery backup when the vehicle is turned off, and you want to use any electronics.
If you accidentally let the battery discharge completely, the alternator will have to work more to charge the big battery. From that perspective, it can have impact on the alternator, but that's a rare case scenario.
PKDM City owner's manual suggest 30 AH battery. I compared it with international owner's manual. Here's what I found.
The max used globally is 40 AH. I think we should stick to this range, since upsizing more will only be a waste of money.