How to detect whether i have arachnodactyly? how long should be the digits?

If they look long...: If the fingers look too long and too slender, one can call it arachnodactyly. Radiologists have a measurement that gives a number: the "metacarpal index" can be determined by dividing the length of each of the last four metacarpals by the width of its midpoint and averaging the values. Marfan's patients are often greater than 8.4, while normals are less than 8.

Unimportant: I am not sure there is a precise standard and long fingers should not be any concern. Check out the huge hands many basketball players have.

Arm span to height: Ratio of greater than 1.05 as one of the skeletal criterion for Marfan's syndrome is the only defined ratio as far as i know. Arachnodactyly is more subjective ie when you see it you know it it lon g slender fingers are not uncommon.