there is no size limit for ppt files, but every wp.com blogger is given 50mb storage limit. so as long as you don’t go over that limit you shouldn’t have a problem uploading the ppt file. there is also an option of purchasing more space.
i’m not sure what’s wrong, try repeating all the steps you’ve done, but change your editor setting from visual to code. in the code mode, after clicking ‘send to editor’ you should be seeing the link tag to your ppt file.
also, you may want to clean up your upload storage via dashboard, select manage, under it select upload.