How does the size of insert affects the rate of Homologous Recombination in yeast?
As pointed out by aandreev, the greater the size of the insert relative to the flanks lower will be the recombination rate. This is because the donor DNA essentially becomes non-homologous. See the figures below:

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  Figure1: Effect of insert size on recombination efficiency. Kung et al. (2013) Appl Environ Microbiol. Mar;79(5):1712-7


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  Figure2: Effect of flank size on recombination efficiency. Kung et al. (2013) Appl Environ Microbiol. Mar;79(5):1712-7


However, you mentioned that you want to make gene knockouts. In this case you just need to make a small mutation (for example introducing a premature stop codon). 

Use of genome editing tools such as Crispr-Cas, ZFN and TALEN increase the rate of transgenesis by introducing DNA double strand break.

These can also be used for making knockouts when NHEJ dominates over HR as the DNA repair mechanism. NHEJ is error-prone and can cause indels that can disrupt gene function.