Hi Nadya,
>Perhaps, I should notify user about an error and give a possibility either fix this or quit
Sounds reasonable, but make quitting sound "good", if bad data can harm later on.
If you are reworking your apps, get the largest database from your customer[s], select max(numfields) and max(len(charfields)) and recalculate probable field sizes. Data can grow.

my 0.02 EUR

thomas