Usually the limits can be increased by changing php.in file. There are two php.ini files in Opencart. One file is for front-end, the second one is for the back-end. For testing purpose you can make changes in both files.
Add/update lines in php.ini file as shown below.
upload_max_filesize = "1000M"
post_max_size = "2000M"
max_input_vars = 5000
suhosin.request.max_vars = 5000
suhosin.post.max_vars = 5000
If the advice did not help, you should contact your hosting support. In some cases, these limits cannot be controlled by users.