Does converting change the model's dimensions?
No, and that's a meaningful difference from a good share of online converters. Many normalise the mesh when they open it — centring it on the origin and scaling it to a standard size — because it previews nicely that way, and hand you back a file with changed measurements without saying so. The ilove3d converter copies coordinates as they are: no rescaling, no recentring, no rotation. If your part was 47.3 mm tall before converting, it is exactly that afterwards, and it keeps its position relative to the origin, which matters when the model is part of an assembly and has to line up with other pieces. That lets you convert something already calibrated and send it straight to the slicer without measuring again. The only tool on the site that moves or scales geometry is transform, and you have to ask it to.
What gets lost converting to STL?
STL is the simplest format the converter handles: it stores a list of triangles with their normals and nothing else. Converting to STL drops per-vertex colours, textures along with their images, materials and their gloss or metalness properties, the scene structure with its names and hierarchy, and animations if the original had any. What survives intact is the geometry: every triangle and every coordinate, in the same units. For single-colour 3D printing that is exactly what you need, which is why STL is still the industry standard three decades on. If instead the model is headed for a website, augmented reality or a 3D engine, use GLB, which keeps materials and textures in one file, or PLY if per-vertex colour is all you need.
Why can't I convert TO GLTF, only GLB?
Because they are the same format with different packaging, and one of the two is clearly better to download. GLTF in its classic form is a JSON file that references its resources separately: geometry in a .bin, each texture in its own image file. That works well on a web server, where each piece is fetched independently, but it is awkward to download and very easy to break — move the .gltf without its neighbouring files and it stops opening. GLB is exactly the same content packed into a single binary file, everything inside. Any program, viewer or engine that opens GLTF opens GLB, so exporting GLB costs you nothing and spares you the classic loose-files problem. That's why the ilove3d converter always hands you GLB.
Does it support 3MF files?
As an input format, yes. You can open a 3MF and convert it to STL, OBJ, PLY, GLB or USDZ, which is the usual case: 3MF is what several modern slicers and modelling programs export, and sometimes you need it in something more universal to share it or print it elsewhere. As an output format it isn't there yet, for a practical reason: 3MF is a compressed container with XML inside that also describes materials, colours and even how parts are arranged on the print bed, and writing it properly is considerably more work than writing an STL. In the meantime virtually every slicer takes STL without complaint, so the 3MF-to-STL route covers what most people actually need.