Adult size
Mexican Red Knees are generally expected to reach about 14–15 cm (5.5–6 in) as adults. Individual Brachypelma hamorii may finish smaller or larger depending on sex and normal variation.
Do not use one advertised maximum as a promise for a Mexican Red Knee. Size figures in the hobby are best treated as useful ranges.
How size is measured
Tarantula size is commonly discussed as diagonal leg span. For Brachypelma hamorii, that gives a rough comparison with other species, but it is not necessary to stretch or position the spider for a measurement.
Estimate size from a normal resting position or during rehousing if needed. Avoid disturbing a Mexican Red Knee purely to obtain an exact number.
Slings and juveniles
Young Mexican Red Knees start far below adult size and grow through repeated moults. With a slow growth rate, Brachypelma hamorii may spend very different amounts of time at each juvenile stage.
Use the current size of Brachypelma hamorii when choosing feeders and housing. An adult-size figure does not mean a sling needs adult-size prey or an adult enclosure.
Male and female size
Males and females of Brachypelma hamorii can differ in build after maturity. Mature males often look leggier and slimmer, while adult females usually remain more robust.
Body shape alone is not a reliable way to sex a juvenile Mexican Red Knee. Use a suitable moult when possible.
Enclosure size follows the spider
Increase enclosure size for Brachypelma hamorii when the current setup no longer gives enough retreat space, substrate or practical access for water and feeding.
Moving a Mexican Red Knee too early into a very large setup can make basic care harder. Rehouse in sensible stages as the spider grows.
Tracking size over time
Keep moult dates and an occasional approximate size for Brachypelma hamorii if you want to track growth. That is more useful than trying to measure the spider repeatedly between moults.
A simple record will show how quickly a Mexican Red Knee is growing and when another enclosure upgrade is likely to be needed.