.THIS .forceCanvasApp {
    position: fixed;
    width: 1000px !important
}

<aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    <ui:button class="slds-align_absolute-center" label="Click Me To Popup!!!" press="{!c.openmodal}"  />
        <div role="dialog" tabindex="-1" aria-labelledby="header43" aura:id="Modalbox" class="slds-modal slds-modal_large">

            <div class="slds-modal__container" >
                <force:canvasApp developerName="CanvasApp" parameters="{!v.identifie}"/>
            </div>
        </div>
        <div class="slds-backdrop " aura:id="Modalbackdrop"></div>

openmodal:function(component,event,helper) {
        var cmpTarget = component.find('Modalbox');
        var cmpBack = component.find('Modalbackdrop');
        $A.util.addClass(cmpTarget, 'slds-fade-in-open');
        $A.util.addClass(cmpBack, 'slds-backdrop--open');
    }