、。
It was a rush job, so the sizes might not match.
Literal
Rush-job was [because-だったから], size [subject-が] not-matching might.
突貫作業 (とっかんさぎょう, 'rush work, crash project') is the compound for hurried, intensive work. だったから explains the cause in past tense. サイズが合わない ('sizes don't match') uses 合う ('to match, to fit'). かもしれません is the polite 'might.' A common workplace disclaimer after hurried work.