Locally repairable codes (LRCs) are error correcting codes used in distributed data storage. Besides a global level, they enable errors to be corrected locally, reducing the need for communication between storage nodes. There is a close connection between almost affine LRCs and matroid theory which can be utilized to construct good LRCs and derive bounds on their performance. A generalized Singleton bound for linear LRCs with parameters $(n,k,d,r,δ)$ was given in [N. Prakash et al., "Optimal Linear Codes with a Local-Error-Correction Property", IEEE Int. Symp. Inf. Theory]. In this paper, a LRC achieving this bound is called perfect. Results on the existence and nonexistence of linear perfect $(n,k,d,r,δ)$-LRCs were given in [W. Song et al., "Optimal locally repairable codes", IEEE J. Sel. Areas Comm.]. Using matroid theory, these existence and nonexistence results were later strengthened in [T. Westerbäck et al., "On the Combinatorics of Locally Repairable Codes", Arxiv: 1501.00153], which also provided a general lower bound on the maximal achievable minimum distance $d_{\rm{max}}(n,k,r,δ)$ that a linear LRC with parameters $(n,k,r,δ)$ can have. This article expands the class of parameters $(n,k,d,r,δ)$ for which there exist perfect linear LRCs and improves the lower bound for $d_{\rm{max}}(n,k,r,δ)$. Further, this bound is proved to be optimal for the class of matroids that is used to derive the existence bounds of linear LRCs.