Is there a limit to how many characters across each line can be in the AppendFile command?.
In "AppendFile" you can specify up to 20 variables that are concatenated by the AppendFile command, using a tab character as delimiter (or the delimiter that you specified in OpenFile). The content of each of these variables can be up to 4000 characters long. If your applications needs larger variables, or more variables, I suggest you use the format

AppendFile "..." template="templatefile.txt"

instead, where "templatefile.txt" consists of one line only that has the following format:

&V[x1]&V[x2]&V[x3]&V[x4]&V[x5]&V[x6]&V[x7]&V[x8]&V[x9]&V[x10]&V[x11]&V[x12]...

Of course you may choose your own meaningful variable names instead of  x1, x2, .... The number of variables is not limited in this case, but the maximum length of the template line is 4000 Bytes. The resulting line can have a length of up to 1MB.