I don't know what the size limit is, but I can tell you it's at least 16GB as I am using an old Amazon Basics 16GB class 10 MicroSD card without issue.
I would avoid Sandisk based on how many comments I see here saying their Sandisk card didn't work, along with my own experience with one I bought after searching "MicroSD card for Roku 3".
Insert the card with the Roku on - not unplugged, not asleep. If it works, it will ask you if you want to format the card...... if it doesn't work, nothing will happen. Without turning it off, unplug the HDMI cable, push the card in until it clicks in, and plug the cable back in.
If you inserted the card with the Roku asleep, chances are it will say it was unable to mount the card and then you'll end up spending money on another card you don't really need (personal experience here).... just insert it with it on and plug the HDMI cable back in and you should be good, if not, take the card out, power it off and back on, then when it's up take the cable out and plug in the card, plug the cable back in and you should be good.
Reviews on Amazon say this works, 16GB for ~$7.48: Amazon.com: Gigastone 16GB Micro SD Card, Camera Plus 85MB/s, Full HD Video, U1 C10 Class 10 Micro SDHC UHS-I Memory Card, with MicroSD to SD adapter: Computers & Accessories
Also this one, 8GB for $5.98: Amazon.com: Gigastone 8GB Micro SD Card, FHD Video, Surveillance Security Cam Action Camera Drone Professional, 80MB/s Micro SDXC UHS-I U1 Class 10: Computers & Accessories