Following command count the size of all files (-type f) newer than 10 days (-mtime -10). The size is printed in megabytes. "%k" argument of printf returns size in kilobytes, but "a/1024" in awk change it to megabytes.
find -type f -mtime -10 -printf "%k\n"| \
awk 'BEGIN {a=0} {a=a+$1} END {print a/1024}'