We have used surface X-ray diffraction (XRD) to measure strain in coherent islands of cobalt silicide formed by depositing 0-5 ML of cobalt on Si(1 1 1)-7 x 7 at various temperatures. Silicide structure is determined from truncation rod scans, while average island dimensions (width and height) and strain are determined from diffraction lineshapes and verified with TEM. At 800 degreesC, the islands are fully coherent B-type CoSi2, approximately 8 nm thick and > 300 nm wide, with atomically flat top and bottom surfaces. At 500 degreesC, the islands are CoSi2, approximately 6 nm thick and 35 nm wide. The lattice parameter is similar to 50% relaxed, primarily due to strain relief at the island edges. At 300 degreesC and coverage of 2 ML, the islands are similar to3 nm high and similar to3 nm wide and show long-range position correlation due to registry with the substrate. At higher coverage, they are similar to4 nm high and similar to 40 nm wide and commensurate, but lack long-range correlation. It appears that the silicide formed at 300 degreesC is pseudomorphic Co2Si-theta for coverage below 2 ML, but is a mixture of CoSi2 and metastable CoSi(CsCl) at higher coverage.