Bulk metallic glass (BMG) composites containing B2-CuZr phase are of interest due to they behave large plastic strain and apparent work hardening in tension. Nevertheless till now most BMG composites containing B2-CuZr phase are based on Cu_(47.5)Zr_(47.5)Al_5 or Zr_(48)Cu_(47.5)Al_4Co_(0.5) BMG, which has limited glass forming ability (GFA). The prepared sample size is small, which restricts their potential engineering structural applications. In this work, Zr-Cu-Al-Y quaternary system is selected due to its high GFA. By tuning composition close to CuZr alloy in Zr-Cu-Al-Y quaternary system, Zr_(46.9)Cu_(45.5)Al_(5.6)Y_(2.0) BMG is selected because it has proper GFA (critical diameter D_c=5 mm) and relatively large fracture toughness (K_Q=(493) MPa?m~(1/2)). By decreasing the cooling rates of the melt via increasing diameter of casting rods, large-sized in situ Zr_(46.9)Cu_(45.5)Al_(5.6)Y_(2.0) BMG composites containing 13% and 25% volume fractions spherical B2-CuZr phase were prepared in the casting rods with 6 and 7 mm in diameters, respectively. In compression testing, the in situ BMG composites containing 25%B2-CuZr phase promote multiple shear bands within glass matrix and remarkable global plastic deformation, accompanied by a large compressive plastic strain as 6.5%. Nevertheless in tension testing no obvious global ductility was achieved, which attributes to the low mode I fracture toughness and small plastic zone size (R_P=88 mum,R_P=(1/3pi)(K_Q/sigma_y)~2) of glass matrix. Three point bending test results show that Y has an adverse effect on the fracture toughness and plastic zone size of Zr-Cu-Al BMGs. In contrast to Zr_(46.9)Cu_(45.5)Al_(5.6)Y_(2.0) BMG, fatigue pre-cracked Zr_(48)Cu_(45)Al_7 BMG plate samples can be prepared and exhibit a high fracture toughness (K_Q=(623) MPa?m~(1/2)) and a large plastic zone size (R_P=150 mum) in plane strain state. Our results show that GFA and fracture toughness of glass matrix should be balanced when designing new BMG composites containing B2-CuZr phase.