Just select the one whose size you want to change, go into the editor and adjust the size until it's right. Bryce defaults to some multiple of its "unity" size when importing regardless of the size of objects when created elsewhere. I never leave objects the size they are when importing. If I'm doing an indoor scene with people, I use 1 Bryce unit = 1 inch as a scaling guideline, so for example, when a character is imported, and it comes in at 40.96 units (a 'unity' multiple) but I want the dude to be 6' (72") tall, I grab an onscreen calculator and figure out what percentage increase I have to make it 72.00 units, then go into the 3D transformations item on the size-adjust menu and apply that number. Voila - it's sized to what I want. Then I make other items and/or characters the appropriate sizes via the same method, and all the objects in my scene are in accurate scale with each other. Then I can arrange them and don't ever have to worry about proportions seeming odd.