Pairing Field size as security parameter I have read Pairings for cryptographers: It states that the groups $G_1$ and $G_2$ are groups of points on the curve and the group GT is a subgroup of the  multiplicative group of a related finite field $F_{q^k}$. l is the order of these three groups. It then states that the sizes for $q$ are more comparable to RSA rather than like RCC. I implemented a type D curve. Lynn states for the type D curve that 170 bit for q (and l) should be sufficient. 

I have problems to understand this all since I am missing some mathematical background. The size of q is very much depending on the type of curve. Or is Lynn's statement just outdated? When I implement a scheme based on this particular "type D"-curve, how do I define the security parameters and how do I justify my decision?

I would have thought, that the group operations in $G_1$ are the fastest and all others are slower, therefore this size matters. But now I think this is too easy and wrong.
Thats a bit outdated as for your choice of setting (with $k=6$) the 170 bit will give you 1020 bit security in the subgroup of $F_{q^6}^*$.
In your case for embedding degree 6 I would at least take 224 bits which would give you 1344 bits in the multiplicative subgroup of $F_{q^6}^*$ or you even go to 256 bits which will give you 1536 bits. Anyways the setting with embedding degree 6 will give you not an optimal efficiency-security tradeoff as you have to take far higher security levels in the curve to achieve acceptable security levels in $G_T$.
The choice of Baretto-Naehrig curves will give you the optimum security-efficiency tradeoff (they have $k=12$) today and they are for instance implemented in the RELIC toolkit.
You may look here what parameters for elliptic curves and multiplicative groups of finite fields are recommended today by different institutions.

Update (2021): As mentioned in the comments, already in 2014 the parameters mentioned above for MNT curves only gave security (far) below 100 bit. For an up to date reference on parameters one can look here: https://members.loria.fr/AGuillevic/pairing-friendly-curves/