Every island has a 200x200 boundary. We know this because of the many players who have islands next to eachother and have connected them. I cannot confirm the same for 149x149 because no one really buys that. (I know at least one island that is 148x148)
Island 1 bedrock to island 2 bedrock is 200 blocks. So each island gets expanded 99 blocks in each direction so every island is 199x199
When would this not be true? To my knowledge each new island starts its bedrock 199 blocks away from the last. Each bedrock spawning in at 200block intervals
Fly in one direction and note the x or z coord (whichever way you fly) and then find the difference. You can also do /block name to find your boundaries