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Since there is an inverse relationship between distance and performance, you must consider not only the size of your office but also desired throughput and total capacity (number of concurrent users x bandwidth per user). For example, research conducted by Atheros found that 802.11a radios carried 5.2 Mbps at a maximum distance of 225 feet vs. 21 Mbps at 65 feet in a typical office composed of conference rooms, closed offices, and cubicles.
To completely cover a larger office will require multiple cells (access points). However, deploying the bare minimum number of cells will result in a lower total capacity than deploying more (narrower) cells, each capable of sustaining higher throughput. With 802.11a access points, higher data rates and more non-overlapping channels allow you to build a faster WLAN with greater capacity than with 802.11b. However, you will not find the reach of an 802.11a access point any greater than a comparable 802.11b access point. RF absorption by obstacles may also be greater for 802.11a than 802.11b.
To design coverage for your office, nothing really substitutes for a thorough site survey. However, here are some estimates on RF signal loss due to obstructions, courtesy of the Planet3 Wireless CWNA Study Guide: dry wall = 5-8 dB, six inch thick solid-core wall = 15-20 dB.
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