I would wait until the weather warms up and new growth appears to see how far back you need to cut them. In your area winter freezed will tend to kill much or all of the topgrowth on a bougainvillea and thus you may wish to try another plant. There are many good options including 'Tangerine Beauty' Crossvine and Carolina Jessamine (this plant is poisonous so it should be avoided if small children are around).
The following website has some good options for Florida landscapes:
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