128gb is the largest size that will work and is what I currently have. The 16gb card that comes with it doesn't even record a full day if there are locked files from emergency or parking events that haven't been formatted for a couple of months. I found out the hard way when I didn't have footage that I needed from just one day earlier because the 16gb size was so small. I didn't realize that it only came with a 16gb card until it was too late. What a joke!
With 128gb, I now have enough storage for almost a month of driving footage even with locked events.
To view the files on the Toyota Dashcam app, you need to make sure you turn off your phone's Bluetooth, as it will try to connect to your car and then your car's Wi-Fi will fight with the dashcam's Wi-Fi and kick you off the camera. You can view the video files on the Toyota Dashcam app, but they are 3 minutes long and take a very long time to appear using the camera's Wi-Fi due to the file size. I use the time stamp to locate an event and just download the clip to the app. Once the clip is actually downloaded to the app you can view it quickly on you phone. As you have found out, it is quicker to view the files on your computer as the thumbnails show up almost instantly so you can find your event.
I also have a FitCamX dashcam and prefer their app over the Toyota one. The size of the clips are adjustable and I have mine set to 1 minute. This allows the thumbnails to show on their app quickly and it's easier to find the event you are looking for. I kept the Toyota cam because it shows speed and GPS coordinates and has parking mode and the FitCamX does not. I like having the redundancy of two cameras, especially since the Toyota cam didn't have the footage I needed once, but the FitCamX did.