I am now new to scripts but i find it really difficult understanding "for" command (especially with that tokens and delimiters etc) . Saying so, i think that for command can be used to do what i am doing. If its not and there is an easier way, ignore my ignorance :(

Say i have multiple lines in a text file abc.txt with each line starting and ending with " (quotes) E.g. a file of 3 lines

"hey what is going on @mike220. I am working on your car. Its engine is in very bad condition" 
"Because if you knew, you'd get shredded and do it with certainty"
"@honey220 Do you know someone who has busted their ass on a diet only for results to come to a screeching halt after a few weeks"

How can i trim each line, within the quotes, to a Fixed length say 30 or 50 or 100 characters (including spaces) I want to enter the number of character in batch and it can trim accordingly and produce a file def.txt with trimmed lines within quotes.

Say i enter 50, results of above example should be

"hey what is going on @mike220. I am working on you"
"Because if you knew, you'd get shredded and do it"
"@honey220 Do you know someone who has busted their"

Thanks

P.S. if you use For command, kindly please explain the command.

EDIT: Though the answer provided worked, there is an issue with non english text. I am getting garbled text in Output file for non english text in input file . Any help @barlop here is the nonenglish text ( 1 line)

"???? ?? ???? ?? ???? ???? ??? ?????? ???"