The correct answer to your question is YES, and NO.
YES, if you are editing a long line in the editor of 128K BASIC, you are limited to the size of line buffer, which can hold only 20 screen rows. You can confirm this by reading the ZX ROM Disassembly which can be found here (note that you will need to be reading disassembly for ROM 0). Let me quote a comment from there, given under the heading "128 BASIC Mode Limitations":
There is a practical limitation on the size of lines that can be entered. It is limited to 20 indented rows, which is the size of the editing buffers. Typed lines greater than 20 rows get inserted into the BASIC program, but only the first 20 rows are shown on screen. Editing such a line causes it to be truncated to 20 rows. There is no warning when the 20 row limit is exceeded.
Just to illustrate this point, I quickly did the following steps:
- I created a line that contains 21 rows:
- I tried to press the return key, to see what gets saved (and frankly it looks fine at this stage!):
- I tried to edit the line again, and then it becomes clear that things are not all rosy:
- To make things properly interesting, I tried adding a space after the fullstop, just before the first word TEST at the end and pressed the return key to obtain
So, 128K BASIC editor is basically inadequate.
NO is also a correct answer, because the actual storage format of ZX BASIC allows lines with lengths up to almost 64K, which in any case cannot be inputted directly from BASIC. I believe that 48K BASIC is free from the described limitation and the line editor of 48K BASIC can only run into trouble saving your line if you run out of available RAM, which in 48K configuration can happen as your line length exceeds something like 20K of characters.