there no known limitation of Anaplan in terms of large files. I've heard frequently people loading files counting in GBs.
I think in your case your mapping is wrong. It's very unlikely you have more than 5000 line items. You must have swapped a dimension and the line items on the first tab mapping.
Since this is a certification related question, community recommends figuring it out and doing it on our own so that we know what kind of things we need to look out for. However as @nathan_rudman already suggested Issue seems to be with the mapping. Even I feel it is so
Here is a tip from my end:
Your Line Items should be mapped as Column Headers or Fixed Line Item, You can map rest.
Per your screenshots, it looks like there are 5000 line items in the module where you are trying to load data. First of all ,is it really true that you have 5k line items. If not, then this mapping is not right. Suggest you to try to map it correctly.
Its against any decent practice to have these many line items in a single module.