The size of 3 kB is mentioned in the latest release of the IEC 62106 standard. I think it is appropriate. For the best user experience, it is important how fast the logo loads, rather than how many graphic details are there. The LCD area which is dedicated on receivers for the logo display, is usually small, so sending larger files only result in longer transfer time.
The P164, however, does not have such technical limitation. It is able to send over 8 kB file from its internal memory or up to 163 kB when driven by external application. The Magic RDS 4 is now restricted to 6 kB, as some space is reserved for other RDS2 features. In my opinion, best results can be obtained with a file size of 3 to 5 kB.
If there is an interest, I can clarify the technical background of why the RDS2 system is not able to meet the original expectations or why 4 carriers may be slower than 2 carriers, etc. The original idea of sending different graphics and large files was very naive. After some experiments made on various stations around the world, the standard has been updated to the present form.
Some logo examples are attached below. They have all about 3 kB. Click for full size.
Typical transfer time of 3 kB file is under 1 minute if FM reception is excellent.
