Is there a maximum number of files per directory or maximum directory size?
This, really, isn't an android question, it relates more to file systems.

The limitation is set by the file system. FAT32 can have 65,536 entries. Each file and 
subdirectory takes from two to thirteen entries, depending on the 
length of its name. 

There is one entry that hold the Short (8.3) name of the file name. If the actual name doesn't fit that entry, then there will be one to twelve more entries, each of which holds 13 bytes of the full name.

The size of the file itself, doesn't have make a difference, unless the file is larger the 4GB, which is the maximum file size for a single file in FAT32.

You can read more on the FAT32 Spec if you wish here.