Help: How to Determine Steel I-Beam Load Strength (temporary support beams/column)

I have an enclosed back porch with a concrete block foundation that must be replaced. Its foundation is separate from the main homes foundation as it ajoins at the ends where the main foundation exists and accessed with a door cut out from main foundation.

The porch is 8 feet wide by 18 feet long, single story with pitched roof,asphalt shingles containing nothing more than a deep freezer, closet spaces.

If I assume the structure`s weight as 50 lbs per square feet (?) what size steel I Beam is safely adequate to use to support the weight of the porch while I remove the blocks and install solid form concrete walls? W6X20, W8X28?