This is based on the following PHP values: [b]upload_max_filesize[/b] and [b]post_max_size[/b]. They both need to be larger than the size of file you are trying to upload.
In the Overview admin section, click on [b]Show PHP Information[/b] and find the values for upload_max_filesize and post_max_size
and take note of the [b]Loaded Configuration File[/b] to know where the [b]php.ini[/b] file is located that is being used. Need to change these values there.
Consult your webhost for more help if needed to increase these values or let you use a custom PHP.INI with your gallery if this is a shared server and they don't want to modify the default values.