The main concerns would be smoke or the full fire plume of flame and smoke with considerable radiated heat.Smoke from a small opening window at the same level of the external escape route is unlikely to be a problem - it is unlikely to make the external route untenable. It would be different if the window was underneath an external fire escape.
In the case of a fully developed fire I would take a look at the CP3 1971 design for balcony approach flats which are a similar concept and certain dimensions were recommended for the width of the balcony and the height of the sill so persons could, if necessary, crawl below the level of the window.
With a full evacuation strategy incorporating detection the fully developed fire is not likely to be an issue during the evacuation phase.
You could consider a vertical screen between the exit door and the window to deflect such a plume, partial secondary fire resisting glazing to the lower part of the window leaving just a small opening vent at the top, or of course sprinklers if you have deep pockets.