Regrettable Shopping

The waist size of them was is 30 inches, that is a little bit large for me.

I have worn the trousers in size of 28 inches since I was a high school student.

I changed the size of the trousers for the first time.

A more natural way to say this is "I changed trouser sizes for the first time."

I regret the shopping purchase.

I should have bought them in size of 28 inches.

I should try to fit my body into them by with diet & exercise.

OR: I should try to fit my body into them by dieting & exercising.

The waist size of them was 30 inches. That is a little bit large for me.

I have worn the trousers in size of 28 inches, since I was a high school student.

I changed the size of the my trousers for the first time.

I regret the shopping. buying them.

I should have bought them in size of 28 inches.

I should try to fit my body to in them by diet & exercise.

Last weekend, I bought a pair of pants.

"Trousers" sounds British to me. I doubt that they use that word in the US, but maybe I'm wrong.

Their waist size of them was 30 inches, which is a little bit large for me.

Beth's correction is also good, "the waist size..."

I have worn size 28 since I was a high school student.

We also commonly use the verb "to be" with size. For example, "What size are you?" "I'm a size 28."

However, I'm getting fat lately. / I've been getting fat recently.

I changed pants sizes for the first time.

I regret the buy / the purchase.

I should have bought them in size of 28 inches.

I should try to fit my body into them by dieting & exercising.