Translation:This sweater is not long.
This is an interesting facet of Japanese pronounciation of the "g" sound. When it's at the beginning of a word, it's a hard G, but the "ga" particle or other G-sounds in the middle of a word sound more like "ng" and are much softer. "Nagai" sounds much more like "Nang-ai" or "nang-aku-nai" in this case." Give it a re-listen with that in mind.
After doing just a little research, it seems the nasal "g" sound was set as the standard long ago. Both the hard "g" and nasal "g" can both be heard today, however. Some people attribute this to social class, age, or region.
So I guess pronouncing it either way will still be understandable, but it's not technically correct according to standard Japanese.
I would think in japanese it would translate back to "i want a longer sweater" ie: わたしはもとながいセーターがはしいでず I don't know if that is how you spell the words in japanese, but that is how I imagine one would explain that they want a longer sweater, which is similar to saying your sweater isn't long enough.
What's the implication of this sentence? Is the sweater the right size, or not??
Implied context 1: Unlike the last few sweaters you tried on, THIS sweater isn't too long for you. Implied context 2: The speaker doesn't want to criticize the sweater or the wearer, but is suggesting that maybe a bare-midriff look isn't appropriate at a formal dinner party.
Or could it be either?