The Story :
This is a very general and simple issue and that’s about your host configuration not theme or plugin files.
In fact many shared hosts put very low limitations on maximum file size upload. so when you try to upload a file bigger than maximum upload limit, you get some error like ”are you sure you want to do that?” and then you have to restart.
In this case you can upload your theme by FTP or increase maximum upload size
So, How to increase maximum file upload size?
Here are configuration settings which will affect upload size restrictions:
upload_max_filesize, which sets an upper limit on the size of uploaded files
post_max_size, which limits the total size of posted data, including file data
max_input_time, which restricts the length of time the script is allowed to process input data, including posted values
These values are set differently on different hosting environments.
How can i change these values?
Many hosts allow you to create your own php.ini file in the root directory of your site. This file can override some of the servers default PHP settings. If not already done simply create a php.ini file and place in the public_html directory of your site.
If the server is configured correctly, after inserting this snippet the servers default max upload will be overridden.
in this example it was changed to 25 megabytes.
; Maximum file size of post data that PHP will accept.
post_max_size = 25M
; Maximum allowed file size for uploaded files.
upload_max_filesize = 25M
What if my host doesn’t apply my php.ini file?
In this case you need to contact your host provider and ask them to increase maximum upload size.