I honestly had so much fun answering your question so here's the results!
The max amount of squares away to hear a whisper: 8 squares left to right, 6 squares up to down (you have to be on the same screen and see the character fully) You cannot hear a whisper if your on a different floor level.
The max amount of squares away to hear a YELL: 31 squares left to right, 31 squares up to down (on the same level) However, if it's -1 level from each other 31 squares still but your on 1 different level. I went to Darashia's depot to test out my other theory of how many floor levels make a difference. So from my understanding it doesn't matter how many levels your on there's no max floor level possibly? I'll look into finding a location that has a ton of levels to test this out again but there's a 31 square difference left and also right up to down. This is just what I tested where I could in darashia depot
I had many other screenshots but I wanted to include just this one example of where I stood and where he stood.
I wanted to include fun this facts about whispering and yelling just because we're on the topic as a bonus:
It is possible to raise or lower your character's voice. To do so click on the little sound icon in the bottom left corner. The default setting is normal loudness. By clicking on the respective selection buttons, you can alter the setting to yell (represented by two sound waves), which means that your character's next statement will be heard over a great distance. Conversely, if you set your local chat to whisper (no sound waves icon), the next statement will be whispered, i.e. it will only be heard over a very short distance. Please note that your character's next statements will be automatically reset to the default setting "say", regardless of the client you are using.
Your character's loudness can also be modified by using shortcuts. These work in an identical manner on both client types: You can raise or lower your voice by pressing the hash key "#" as well as a letter that will specify the loudness of your character's statement in front of the text. Thus, "#w [Text]" will cause the following message to be whispered, while putting #y in front of the text will result in a yell and #s will make your character say the following text normally. Note that messages that are entered after #s, #y or #w are always posted in the local chat channel, regardless of which channel is open while it is entered. Also, keep in mind that there is a delay of 30 seconds on yelling, so characters cannot yell more than 2 messages per minute.