Vowel length isn’t as important in English because of our wildly different accents. You learn to tell the meaning through context.

For cease and seize, cease is like ‘see’ with an s sound at the end. You say it through your teeth with your lips pulled back, as if you were grimacing. Seize is similar to ‘sea’ with a z sound at the end. You pronounce it with your teeth together still, but this time with your lips relaxed instead of pulled to the sides.

For bag and back, the a in bag is longer than the a in back. Although if you pronounced them interchangeably, people would still understand you because of the context from the situation.