With ADS-B, there are two NEXRAD products; regional and CONUS. Which one you get will depend on the tower type that you are connected to and receiving from.
Hi-altitude and Medium-altitude tiers of ADS-B towers transmit the CONUS product for the lower 48 states This radar imagery has a large pixel size.
Regional radar is provided by each ADS-B tower, but the extent to how large of a radius of radar imagery is shown is determined by the tower type (low tower - 150 NM radius, medium tower - 200 NM radius, high tower - 250 NM). Depending on which towers are being received, a mosaic is assembled with the regional and CONUS products.
For regional radar, the NEXRAD images are updated every 5 minutes. ADS-B transmits the images every 2.5 minutes, but the data only changes on a 5-minute schedule. In clear air conditions, the update rate is once every 10 minutes.
For CONUS radar, the NEXRAD images are updated every 5 minutes, but the images are only transmitted once every 15 minutes.