This is the 11410th most frequent Russian word.
длиннющий
Extremely long.
The word "длиннющий" here emphasizes the extreme length of the garden, beyond standard "long".
У него такой длиннющий сад, что до конца не видно.
He has such a long garden that you can't see its end.
In this context, "длиннющий" highlights the exaggerated length of the shopping list, likely implying a lot to prepare and buy.
Она написала длиннющий список покупок перед праздником.
She wrote an extremely long shopping list before the holiday.
"Длиннющий" in this sentence conveys the impression of a tremendously long corridor, potentially adding a dramatic feeling to the journey.