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We propose a unified framework for segment quantization of speech at ultra low bit-rates of 150 bits/sec based on unit-selection principle using a modified one-pass dynamic programming algorithm. The algorithm handles both fixed- and variable- length units in a unified manner, thereby providing a generalization over two existing unit selection methods, which deal with single-frame and segmental units differently. The proposed algorithm performs unit-selection based quantization directly on the units of a continuous codebook, thereby not incurring any of the sub-optimalities of the existing segmental algorithm. Moreover, the existing single-frame algorithm becomes a special case of the proposed algorithm. Based on the rate-distortion performance on a multi-speaker database, we show that fixed length units of 6-8 frames perform significantly better than single-frame units and offer similar spectral distortions as variable-length phonetic units, thereby circumventing expensive segmentation and labeling of a continuous database for unit selection based low bit-rate coding.