Master bath conundrum

Hello everyone, I was just looking for advice on the size of beam I would need to replace a load bearing wall in my basement. I've found a plethora of information on how to go about doing this renovation but there's no clear cut guidelines for the sizing of the beam as far as I can find so any and all help would be greatly appreciated. To start the house is 2 story brick house with cinderblock foundation and concrete slab basement floor. The basement is separated by one long 2x4 support wall with a doorway that runs perpendicular to the floor joists. The joists overlap by at least a foot over this wall. The wall is raised on a row of cinderblocks, I'm assuming to help bear the load of the house. I want to remove a 16 or 18 foot section of this wall to join two rooms together. Directly above this section of the basement is the living room and kitchen which also has another wall in the same location which I assume is also load bearing. Directly above that is attic space. Would anyone be able to give me a decent idea on what size beam I would need to go with for this project? Also which material would be the best/most cost effective solution, traditional lumber, LVL, or steel beam? Sorry for the long post. Thank you in advance for your input. If there is any more information that you need let me know.

Cottage is in northern Maine and has been used every summer but not updated since 1960. It sits on cement blocks with a dirt floor crawl space underneath. If you cut a hole in floor, you would see dirt about 20 inches down. Electric baseboard for spring and fall, but no heat / power all winter. Previous owner had carpet over existing oilcloth when cabin was built in 1910. I plan on pulling up the carpet and oilcloth and seeing what is below. The walls are finished with a wood panelling (see picture) and apparently insulation batts (1960) - not sure about vapor barrier but I assume not. Different color panelling is used on different walls. I plan on removing all panelling, but am unsure what I should replace it with. My wife would like the walls painted for more of a house feel than cottage. I assume drywall is not the best option due to temperature drops and humidity. Could I install similar panelling, or paint over existing once I see what is going on in behind. What would look good and hold up... On exterior walls, should I install a vapor barrier? I'm not sure where the plastic should go given the temperature changes from summer to winter... help... [ATTACH]55854[/ATTACH]