You should get core loss chart from core datasheet......it shows the power/m^3 for your particular delta B and at your frequency.
Generally you shoudl not go above 300mT in delta B....so for you its +/-150mT...
..but you can exceed if you have fans or only short interval times at max load.
Also remember not to saturate, so dont go too far above 320mT if you can help it.
Small core is good as the volume is low, so the overall dissipation is low, as their is less material to heat up, and because it is small, all parts are close to the core surface, so to speak.
Some people say the hotter it gets...the more convection current tend to flow, which cools it back down again!