Accessing Variables from other functions - Symfony2

Newbie question alert...

Ok so currently trying to Convert a user inputted value (a mass) from what ever Measure they wish to use, into Mg and then store it in the database.

Currently I have this:

public function setUnit($unit)
{
    $this->unit = $unit;

    return $unit;
}
public function getUnit()
{
    return $this->unit;
}
public function setSize($size)
{
    $unit = $this->unit;

    $this->size = $size = new Mass($size, $unit);
    $this->size = $size->toUnit('mg');

    return $this;
 }

The "new Mass" function is something created by TriplePoint for converting from measure to measure.

https://github.com/triplepoint/php-units-of-measure/

Now if I explicitly set the $unit variable to 'grams' and 'kg' or what ever I choose It will do the conversion however I don't want to force the user to use just one measure obviously. So I thought creating a new variable $unit and the setters/getters to go with it, the text entered to the unit box would then be accepted in the newMass function. Instead I get this:

 Unknown unit of measure ($unit)

Which I think means it is trying to use '$this->unit' as the variable and not the text in the text field.

Like I said I am really new to this stuff, sorry if I annoy anyone with this question.

Once I have this in place I wish to create an array for the user to choose from I think I can manage this myself though.

EDIT--

public function buildForm(FormBuilderInterface $builder, array $options)
{

    $builder
        ->add('prod')
        ->add('product_code')
        ->add('size')
        ->add('cost')
        ->add('discount')
        ->add('foodcat')
        ->add('unit', 'choice', array(
'choices'   => array(
    'g'   => 'G',
    'kg' => 'Kg',
    'mg'   => 'Mg',
    'oz'   => 'Oz',
    'lb'   => 'Lb',
),
'multiple'  => false,
));        
}

This is the code for my form.

So I wish the value of $unit to be what ever is selected from the unit choice array. Which is why I am trying to load it by using:

public function setSize($size)
{
    $unit = $this->unit;

Though I am thinking this is not the correct way to get the set variable from "public function setUnit"

Edit -

If I do :

        $unit = 'g';

    $this->size = $size = new Mass($size, $unit);
    $this->size = $size->toUnit('mg');

    return $this;

It works perfectly fine.

So really all Im asking is.. how do I access the variable set in the setUnit method?