Long vowels change words
Vowel length can distinguish Japanese words. Check both the spoken length and the actual kana spelling to avoid confusing what you hear with what you need to write.
One mora or two
おばさん means aunt or middle-aged woman; おばあさん means grandmother or elderly woman. The latter holds the vowel in ば longer. When practicing, count a long vowel as two timing units, or morae.
Lengthening a vowel does not mean making it louder. Practice お・ば・さ・ん and お・ば・あ・さ・ん with a steady beat, then keep the length distinction when you return to a natural speaking speed.
Check spelling as well as sound
Katakana commonly marks a long vowel with ー, as in スーパー (supermarket). The mark extends the preceding vowel: スー has a long u, while パー has a long a.
Hiragana spellings vary, as in おかあさん, おとうさん, and おおきい. おう and えい are often pronounced as long vowels, but do not apply that mechanically to every sequence. Check each word's spelling.
Listen for the extra beat
Listen to a word, write it in kana, and compare it with a dictionary. Hearing a long vowel alone does not tell you whether to write う or お. Check pronunciation and spelling as two separate steps.
When comparing your recording with a model, first listen for any missing long vowels. Morae are not all exactly equal in natural conversation, but removing a length distinction can make a word sound like a different one.
Check: Does おばあさん keep the same meaning if you shorten it to おばさん?
Show the answer and why
No. おばあさん and おばさん are different words. Learn vowel length as part of the word, not as an optional pronunciation detail.