Beam thickness / beam spacing for a walkable intermediate floor?

Hello everyone.

I have been reading passively for a while, but this is my first active question. I would like to install a walkable intermediate floor in our house to store various things. The room over which the intermediate ceiling is to be constructed measures 350cm x 950cm. I imagined this with beams (resting on joist hangers?), spanning the short distance (350cm). On top of that, OSB floorboards or similar. The walls to which the beams are to be attached are masonry on one side and concrete on the other. Additionally, there are two window arches and two door openings in the walls (is this a problem if beams are attached above them?). The question now is how thick the beams need to be dimensioned and at what spacing they should be laid? I still have several rafters from an old attic available with dimensions of 150x100mm. Would this work?

Thank you very much for your help, if I have formulated anything unclearly, please just ask.

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Thank you for your response, but this only helps me to a limited extent. I have considered creating pockets in the masonry wall to make a support, and for the concrete wall, I will use joist hangers. However, I would be very grateful if someone could help me with the beam spacing and the thickness of the OSB panels. The whole thing is intended to be used as an attic/storage space afterwards, so it will not be excessively loaded. A drywall ceiling, simply clad, will be attached to the bottom of the beams.

Warning, I am not a structural engineer. But if I were you, I would use the specified beams and install them at intervals of 40-50 cm. I would not use OSB but rather 25mm thick tongue and groove planks.

I would not trust the beams, as I do not know if the walls are load-bearing or crumbling and held together only by the plaster. I would support the side beams on the walls downwards (like a soccer goal), lay the beams resting on them at 50cm intervals, and put floorboards 23mm or thicker on top.

Thank you very much for your answers! You help me a lot. Why floorboards and not OSB?