[SalesForce] Lightning:select size of biggest option

Setting the options doesn't actually cause the rendering to happen. So, the value you're setting is ignored because it isn't valid at the time. The solution is to use another callback post-rendering. Here's an example app I wrote that demonstrates this:

<aura:application extends="force:slds" controller="LightningAccountController">
    <aura:attribute name="accounts" type="Account[]" />
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" />

    <lightning:select label="Choose 1" name="a_opt" aura:id="a_opt" onchange="{!c.change}">
        <aura:iteration items="{!v.accounts}" var="account">
            <option value="{!account.Id}">{!account.Name}</option>
        </aura:iteration>
    </lightning:select>
</aura:application>

// Controller
({
    doInit: function(component, event, helper) {
        // Request from server
        var action = component.get("c.getAccountList");
        action.setCallback(this, function(result){
            var accounts = result.getReturnValue();
            console.log(accounts);
            component.set("v.accounts", accounts);
            // Let DOM state catch up.
            window.setTimeout(
                $A.getCallback( function() {
                    // Now set our preferred value
                    component.find("a_opt").set("v.value", accounts[4].Id);
                }));
        });
        $A.enqueueAction(action);
    }
})

// Apex class
global class LightningAccountController {
    @AuraEnabled global static Account[] getAccountList() {
        return [SELECT Name FROM Account LIMIT 10];
    }
}

I would expect components in the lightning namespace to match the LEX design in the future, even though we can't say when/if this will happen.

In the meantime, the guys at Appiphony have made a great job creating components that match the SLDS look & feel.

Have a look at the Strike components, in this case the Strike Select one, which is exactly what you want. These are free and open source. In the future, when lightning:select will be matching the LEX UI, you'll just have to replace the strike component by the standard one.