I cannot tell what you are asking here. Please explain your question in a bit more detail.

Hi, I want to create 2 set of alphabet in the font
first for regular text and the second one for footnotes
in order for them to get equal text color the footnote versione should be slightly darker than the regular.

i want the font to use the reguler version for range between 11-14 Pt
and to use the darker version for 6-10 Pt

how do i define this using the feature SIZE????

Do a search for "size feature", there are several old threads that cover SIZE in great detail.

In an earlier thread, John Hudson wrote:
Also, in order to build ’size’ feature data into a font, one needs to use an Adobe FDK-based tool for OpenType Layout tables. A tool such as VOLT, used by pretty much everyone making fonts for complex scripts, which is based around the normal structures of GPOS lookups, has no way to build a ’size’ feature.