If you also run into this problem, dont' worry, here is the solution for you.
First:
In Modx, go "System settings" --> Search "Maximum upload size" --> edit the value (in bytes), I set to "100000000" (= 100 MB)
Second:
In the root of "FileSystems" create a new file called ".user.ini"
upload_max_filesize = 100M
post_max_size = 150M
memory_limit = 200M
Note: post_max_size should greater than upload_max_filesize; and memory_limit should greater than post_max_size.
In some posts, they suggests change those settings in .htaccess. But for me, it doesn't works well in modx.
So use .user.ini file instead.