Here's T2Laser with a large image, it's 210 x 300 mm (1000 x 1500 px). I believe it was 1000 speed and 0.2 resolution.
It generated over 1.5 million lines of G-Code and would take 5 hours and 16 minutes to engrave in grey scale.
You do need to keep the image size reasonable so a 300 x 400 mm image would probably need to be engraved at 0.2 or 0.25 and it's best to resize the image before loading (say 1500 x 2000 px).
The trial is fully functional so you can test the image but you won't be able to generate or engrave the complete job.