Moving files based on size and moving files based on changes to them are very different problems, the former being the easiest to solve. You can write a batch file to run every x amount of time (via task scheduler) that will scan the directory and copy files if they are smaller than a certain size. You might create a file called "size.bat" within the same directory
FOR /F "usebackq delims=" %%A in (`dir /a-d-h /b`) do (
if %size% LSS %maxsize% (
copy "%%A" "C:\Dropbox"
The above file will scan the current directory (for only files, omitting the scanning of any subfolders) and will copy any files it finds to be less than 50MB (which equals 52428800 bytes) to the C:\Dropbox directory. Obviously, you will have to change that directory to wherever your Dropbox directory currently resides. And, you can change the maxsize as well. Just make sure you translate the size into bytes as that's how the "dir" command natively reports back file sizes.
You can then use Task Scheduler to set up this batch file to run at whatever interval you'd like to transfer the files.
The latter issue of copying files based on changes is a much more involved issue that I don't currently have the time to address, but it's doable with the same concepts and coding I've shown you here.