。
Please hold the bat a little long.
Literal
Bat [obj-を] longish [adverb-に] hold [please-ください].
長め (長 'long' + め, a degree suffix meaning 'somewhat / relatively X') is one of a family of degree suffixes (短め 'shortish,' 早め 'earlyish,' 多め 'a bit more,' 少なめ 'on the few side'). The adverbial に makes the compound modify 持つ. This is a stock instruction in baseball coaching — 'choke up on the bat' in its opposite, 短めに持つ.