Once I tried to upload 40k records from 10 Mb Excel file, and had error, can't remember the message, but nothing about size, something useless. It uploaded OK when I split the table on several parts, and it seems like key factor was the file size, some value between 2 and 3 Mb (it might differ in different systems).
I think you can split your file in 2 parts and try to upload each part. Import UI allows to append data instead of creating second table. Also, it's not hard to join if you upload the data into separate tables.
I used Libre Office, where selection can be saved as csv. In MS Office, you can save whole table as csv, duplicate the file (by copy-paste) and edit each of them by notepad, just choose any record at the middle and remove upper and lowed half in different files.
I thank you very much for your response. I did have to manipulate the data to make it a much smaller file. I was then able to upload to a new base. I wish the pro account allowed for larger files. Or at least indicated what was the maximum megabytes. It never actually sent the file was too large, but that was obviously the problem.
I think that limit is not plan-related, because I have the same on Enterprise plan. And it looks not like programmed way to disallow upload after some file size, rather a bug when some of the available resources overloaded by amount of data. Since that happens not very often, maybe it's not a high priority issue.