A 250 watt seems like a lot for a box that size. I have a 125 watt in a 2x3 38" tall brooder and it's plenty warm (and uses lots less power) The temperature chicks need will vary. Put the lamp to one side or the other not in the center so they can have a cooler spot.
You will know if the box is too hot becuse they will avoid the area right under the lamp. If it's too cold they will cheep loudly and pile up on to of each other to keep warm. If it's just right they will use the whole box, cheep quietly and just sleep where ever they happen to drop.
If it's too hot raise the light a few inches every 1/2 hour until it's right, if they are cold lower it..
Our chick where hatched on Thursday and I pre-warmed the brooder to 90 degrees so I could adjust the lamp up and down easily. The brooder is 87 degrees under the lamp and 83- 84 degrees offside. This is 8-10 degrees cooler than recommended in books butslightly warmer that I started out with last years chicks. I will drop the temp 4-5 degrees every week. Though I put them outside in a covered run on warm days I bring the chicks in at night and usually keep it at 70 (maybe 65) degrees until they are well feathered and the outside night time temp is warm enough to put them into the coop with the ladies.