Is there a limit to the number of columns or rows when using the Data Grid Widget?

No, there is no explicit limit placed on it, but I would advise to take into account the forms usability.

Usually the wider the form must be, the less simple to use it is, so I would suggest setting it up so that it has multiple widgets rather than one holding all of the data in it.

So having say 6 columns with 100's of rows is fine, right?

What is the easiest way to import a csv into DGW?

Yes, that is correct :)

Unfortunately, for your other question, the only way to do it would be to add the data manually.

You should add it by holding data of one row on one line where each cell is separated by a comma.

Each time you want to go into a new row, you just press Enter / Return and that new line will mean that the data goes into the next row.