First, disable automatic translation of autoResizingMasks to constraints.
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						[myView setTranslatesAutoresizingMaskIntoConstraints:NO];  | 
					
Next, add the constraint for the fixed width.
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						[myView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:[myView(==200)]"                                                                options:0                                                                metrics:nil                                                 views:NSDictionaryOfVariableBindings(myView)]];  | 
					
Finally, add the contraint for the fixed height.
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						[myView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[myView(==300)]"                                                                options:0                                                                metrics:nil                                                 views:NSDictionaryOfVariableBindings(myView)]];  | 
					
Note that I am setting a fixed width of 200 and a fixed height of 300.
Important note: Since we are adding our constraints programmatically, we need to do it in our UIViewController’s method:
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						- (void)updateViewConstraints  | 
					
(as stated in https://developer.apple.com/library/ios/featuredarticles/ViewControllerPGforiPhoneOS/AdoptingaFull-ScreenLayout/AdoptingaFull-ScreenLayout.html)
Also, remember to call [super updateViewConstraints] when implementing the above method.
EDIT:
There is another way to define fixed width and height using constraints programmatically.
We can use NSLayoutAttributes to define this.
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						[myView addConstraint:[NSLayoutConstraint constraintWithItem:pageView                                                    attribute:NSLayoutAttributeHeight                                                    relatedBy:NSLayoutRelationEqual                                                       toItem:nil                                                    attribute:NSLayoutAttributeNotAnAttribute                                                   multiplier:1.0                                                     constant:300.0]]; [myView addConstraint:[NSLayoutConstraint constraintWithItem:pageView                                                    attribute:NSLayoutAttributeWidth                                                    relatedBy:NSLayoutRelationEqual                                                       toItem:nil                                                    attribute:NSLayoutAttributeNotAnAttribute                                                   multiplier:1.0                                                     constant:200.0]];  | 
					
References:
					