There is an attachment size limit in Webmail. If you wish to send an attachment larger than the limit, you can simply use email software/app to send emails where the only limit is 50MB total email size with no limitation on individual attachments. Please see here
for details of how to set up your email software/app.
Or if you wish to continue using Webmail, you could use a 'zip' facility such as Windows 'send to > compressed (zipped) folder' or WinZip to compress the attachments into a zip file. This can reduce the file size significantly.