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_47.5Zr_47.5Al₅ or Zr₄₈Cu_47.5Al₄Co_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_46.9Cu_45.5Al_5.6Y_2.0 BMG is selected because it has proper GFA (critical diameter D_c=5 mm) and relatively large fracture toughness (K_Q=(49±3) MPam^1/2). By decreasing the cooling rates of the melt via increasing diameter of casting rods, large-sized in situ Zr_46.9Cu_45.5Al_5.6Y_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 mm, R_P=(1/3π)(K_Q/s_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_46.9Cu_45.5Al_5.6Y_2.0 BMG, fatigue pre-cracked Zr₄₈Cu₄₅Al₇ BMG plate samples can be prepared and exhibit a high fracture toughness (K_Q=(62±3) MPam^1/2) and a large plastic zone size (R_P=150 mm) 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.