+(int) minHeight;
+(int) minWidth;

Class *class = NSClassFromString(childClassName);
CGRect frame = CGRectMake(0, 0, [class minWidth], [class minWidth])
MotherClass *view = [[class alloc] initWithFrame:frame];

Class<MyCoolView> class = [NSClassFromString(childClassName);
CGRect frame = CGRectMake(0, 0, [class getMinWidth], [class getMinWidth]);
MotherClass *view = [[class alloc] initWithFrame:frame];

#import <Foundation/Foundation.h>

@interface MyChild
+ (int) minHeight;
+ (int) minWidth;
@end

@implementation MyChild
+ (int) minHeight { return 100; }
+ (int) minWidth { return 300; }
@end

int main(int argc, const char *argv[])
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSString *className = [NSString stringWithString: @"MyChild"];
    Class theClass = NSClassFromString(className);
    NSLog(@"%d %d", [theClass minHeight], [theClass minWidth]);

    [pool drain];
    return 0;
}

2011-08-10 18:15:13.877 ClassMethods[5953:707] 100 300