The coordinate system in Roblox and Blender are different. A meter in Blender is a stud in Roblox. If the mesh exceeds 2048 studs it will be resized. Your mesh should be at 0,0,0 in Roblox. I would need more information like what file format you used .FBX or .Obj and dimensions of the model.
Step1 create a copy of ur blender file (just in case)
Step2 select all ur meshes and press ctrl+j now all ur mesh are one pice
Step3 open studio and import it in there and a dialog box should appear asking for location data and thing so click no , click no
Ur done
To do that, you need to export as FBX. Once you that, you go to Roblox Studio and import by 2 ways:
Number one, import by using the Asset Manager in Bulk Import or number two, import by using MeshPart. Once you do that, I believe you’re going to see it in there.
If you’re having some errors importing them, try watching this (video) from the Roblox YouTuber developer RoBuilder