How significant is organic cotton agriculture to the USA's total global ecological footprint?
The ecological footprint is the impact of a person or community on the environment, expressed as the amount of land required to sustain their use of natural resources.  A global hectare (gha) from Wikipedia:


  One global hectare represents the average productivity of all biologically productive areas (measured in hectares) on earth in a given year.


and...


  The global hectare is a useful measure of biocapacity as it can convert things like human dietary requirements into a physical area, which can show how many people a certain region on earth can sustain, assuming current technologies and agricultural methods. It can be used as a way of determining the relative carrying capacity of the earth.


and ...


  The total number of global hectares is approximately 11.3 billion, averaging about 1.8 global hectares per person (2004).


I highly suggest you read this page here:
http://www.footprintnetwork.org/en/index.php/GFN/page/frequently_asked_questions/ 
and here:http://www.footprintnetwork.org/en/index.php/newsletter/bv/humanity_now_demanding_1.4_earths 

which explains global footprint quite nicely and puts it into perspective.


  The data show that humanity’s demand on the biosphere for providing natural resources and absorbing carbon dioxide emissions is 44 percent more than what nature can provide. This ecological overshoot means it now takes approximately 18 months for the Earth to regenerate what we use in one year. The urgent threats we are facing today - most notably climate change, but also biodiversity loss, shrinking forests, declining fisheries and freshwater stress - are symptoms of this trend.