size does matter (mostly in rendering times). The following thread discuss how scale matters with renders...

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2739056

100 miles ?(I hope you have a professional renderfarm).....

right  off the bat ...it would be unlikely you woul be able to populate a 100 mile stretch of land with trees....(Im having problems with just a small moutain range)...maybe you render it out as a tilable matte painting?

good luck

Quote - Hello,

I´m starting to experience the wonders of the dem files and i´ve been asked to make a flight by a hundred of miles river wich lead to this question. How can i solve the scale issue? Do i work my model in the real scale or shall i reduce it ? What´s the best way to work with extensive dem files?
Thanks