I have a raster data with spatial reference GCS-WGS-1984 (datum D-WGS-1984). Its cell size is mentioned as (0.00045, 0.00045) in degrees. How can I convert this resolution in meters (i.e. linear unit)?
Easiest way is to convert the raster to a projected system, and get the cell size directly in meters.
That not being possible, or desirable, you can still convert lat/lon values to metric values, but you'll run into an accuracy problem. This is because, 1º of distance represents different values in metric units depending on your location and bearing (0.00045º is about 50m around the Equator, but about 30m around London). There are ways to calculate it, but you'll be generalizing somewhat your results.
If accuracy is not very important and you only want an overall idea of the cell size, you can either do it with the Haversine formula (easier but less precise) or Vincenty's inverse formula (more precise, but way more complicated).