Chen Soon Lee Calligraphy Brush - Mixed Fur Brush (Baiyun Series) it comes in 3 sizes catering for different wording dimention.
for the smallest it is for wordings from approx. 5cm x 5cm to 6cm x 6cm.
medium is for approx. 6cm x 6cm to 7cm x 7cm
while the biggest one can reach to approx 7cm x 7cm to 8cm x 8cm.
Some people who prefer to apply more force might able to write bigger characters then the estimated sizes, however it comes with the price of the brush lifespan.
while some others who able to control the brushes well will be able to write smaller characters then the estimated sizes. thus the guideline is mainly for an estimation as different people has a different habit in using a brush.