You are probably running into restrictions from your php.ini file.
There are three basic settings that can limit the upload file size:
You would have to change these three values in your php.ini file to allow the file size you are trying to support. Resizing large files takes a lot of memory, so:
upload_max_filesize = the actual limit of file size you want
post_max_size = has to be larger than the upload max file size to allow the rest of your form fields to be posted as well
memory_limit = probably 5x the upload max file size if you will be resizing the image