Are you reallly asking about DOWNloading or is it about UPloading?

I don't know of any download size restrictions.

I guess you refer to the default memory limit set by Elgg of 64 megabyte. If you need to increase this value, you can modify the value set for memory_limit in Elgg's .htaccess file in the root folder of your Elgg installation. If you need to increase the value for Tidypics thumbnail creation to work, you might also want to check if the values for upload_max_filesize and post_max_size in .htaccess suit your requirements (upload_max_filesize = maximum filesize of uploaded images; post_max_size >= upload_max_filesize x number of images you allow to be uploaded with Tidypics in one batch).

Suggestion: if you have either the Imagick php extension or the Image Magick package installed on your server, you can select either of these options in the Tidypics plugin settings to be used as image library as both have much lower memory requirements (and provide better resize quality results, too).