Step 1: Identify the size of your OEM wheels/tires. If you have a 2019 Accord with 17" tires... that should be 225/50/17 mounted on 17x7.5" wheels with a 45mm offset.
Step 2: Go to an online tire/wheel calculator. In the past, I've used
wheel-size.com.
Step 3: Plug in your OEM wheel size. Then plug in the specs of the 19" wheels that you purchased and play around with the various tire sizes until you find one that the online calculator says will fit while also maintaining the same overall diameter (wheel+tire) so that your speedometer/odometer reading will still be accurate. It doesn't have to be 100% the same- but it should be pretty close... maybe within 1% of OEM spec.
Since you bought wider tires for the rear, you'll have to do this twice- once for the front tires and once for the back tires.
As for smoothness of the ride- it's going to be tough considering the fact that you are going from 17" wheels to 19" wheels- which will be low profile and will ride a bit rougher.
It seems like you have some decisions to make... and it might not be possible to get everything that you are hoping for. The fact that you purchased 19" wheels (with wider tires in the back) seems to indicate that performance is what you might be after. With that said, putting wider tires on the back of a FWD car with a tiny 1.5t motor with less than 200hp sort-of contracts this. If smoothness is your goal- I'd make sure that you go with a high quality tire that is known for its smoothness. It's unlikely that it will be as smooth at your 17" OEM setup though.
Better yet- go to your local tire shop with the specs of the wheels that you purchased and see what they say. A qualified wheel and tire shop should be able to work with you and find something that will hopefully be close to your goal.