which is why the next is better:

\definemeasure[fontsize:big] [17pt] \definemeasure[fontsize:small][0.7\measured{fontsize:big}]

\definefont[big] [name:arial at \measure{fontsize:big}] \definefont[small][name:arial at \measure{fontsize:small}]

\starttext \big Dies ist ein großer Text. \blank \small Dies ist ein kleiner Text. \stoptext

Best wikify this!

Alternatively you can set up symbolic sizes (like we already have a b c d), so that

\definefont[big] [name:arial sa a]

etc can be used

Hans

Thank you! I will wikify it, hopefully in the next days. Huseyin